Hub3: The route(r) of all my problems

Get hold of the installation manuals and repeat the process as you did when the kit was new.

There's absolutely no reason why you should need to replace those devices.

The classic way to replace a Hub is to note the SSID and password of the old Hub and set the new Hub to be the same. All the WiFi devices should then just work. But you were probably not advised in advance to do this. 

You will then need to reset all your WiFi devices to use the SSID and password to the new Hub.  If this doesn't work it's always possible that the replacement Hub 3 is also faulty, as VM has not been issuing new ones for a long time and it will be a recycled one returned from someone else and may not have been thoroughly tested.

But please don't buy new devices if the old ones were doing what you needed before the Hub change.

Also perhaps the old Hub was setup with a split SSID?  It may help the IoT kit connect better.

VM go to SO much trouble with random password to log in the dam hub WHY not make the user setup the SSID and password then a default. 

It is a bad idea to use default wifi and reuse them on another hub because that means as slim is it might be someone knows your wifi and so does VM! 

I always demand they send me the latest routers when renewing contracts. I know if you want seamless crossover its not ideal but given you get locked into 24 month contracts providing the latest hardware isn't a big ask. Plus if you later find you need a newer feature or similar getting a new one sent out is like getting blood from a stone in regards to VM.

They could be running on different ip subnets. Quite a common problem when introducing new routers/network hardware/systems. Check what your camera/smart device subnet is so common ones are 192.168.0.1 or 192.168.1.1. If they are trying to operate on different subnets they won't work at all.

Brevitas est claritas !

the hub will have come with both wifi bands set to the same ssid and password most stuff will work out which is the best signal and connect to which band it wants but some get confused and see the 5 band first and simply get confused - just split them - rename either so you have 2 seperate bands then point the smart devices to the 2.4 band - they should then work - once you have done that you may be able to go back to to joint bands but if you do obviously neme the 5gig to whatever you named the 2.4 to - if that works fine if not stay with seperate bands 

as to some of the other advice - ignore it 

Hey MyHeadsHurts, thank you for reaching out and we are so sorry to hear about your Google Devices, also a warm welcome to the community.

To confirm which devices are these?

Has any of the advice helped at all?

Many thanks for everyone's replies. Much appreciated.

I have only this morning been notified by email that I had any replies at all. So, apologies for not updating sooner.

I will certainly have a read over each and every response. But in the meantime I'll give you a rundown on where I'm currently at:

I can confirm that my external Google cam (battery) is back working fine. Connected it to the 2.4 wifi as before. Worked first time. No issues. Happy days.

Not such good news with my Google nest displays (clocks). I seem to be having multiple issues with these. Sometimes I'll get past one issue and face another. Other times I won't even get as far during the setup stage. It should be noted that both of these devices were previously connected to the 5.0 wifi.

The nearest I've gotten to completion of setup is to successfully connect both these devices to the wifi but then be faced with "Link your account" on each display. I realise that both of these devices require this and obviously the cam does not. So, unfortunately this is now my main issue. I've got to a point where I've told Google support I no longer require their help as I've already spent far too much time on this, without success. But what frustrates me the most is that these devices have been working perfectly fine for, I'm going to say 3yrs at least: never an issue. Until I changed the VM Hub.

As far as I understand, within the Google Home APP, when I click on the device/display which is already connected to the wifi. Long pressing it brings up a 'volume screen'. On this screen I'm supposed to press the 3 dots in the top right corner and there should be 3 drop down options: 'device settings', 'help' and 'feedback'. There is no 'device settings'.

So, I'm unable to get into the device settings of either display device to link it to my Google account. Weird. Even those at Google support can't figure it out. I've tried multiple factory resets of these two devices, starting from scratch. Often, I'll end up at the same point. Unable to link the device. Other times, I'll face a host of different issues. Where on the display of the device it reads "connected" (having previously connected it to the wifi, it seems to keep this setting), but on the Google Home APP, it cannot find the device! I'm not a million miles away from the device either. I'm stood right next to it during setup and, like I've said, it's worked perfectly well in another room of my house previously.

One error message I am now getting more often that not (after factory resetting both devices) is a screen on the Google Home APP which reads: "Connect to same Wi-Fi" "...your device was setup successfully but can't be found. Make sure they are on the same Wi-Fi network" (which I can assure you that is what I'm trying to do. My phone is on the exact same 5.0 wifi as I'm trying to connect these two displays). "Turn off AP isolation" ????? "If AP isolation is enabled on your Wi-Fi router, you will need to disable it." I have no idea where this setting is and 100% do not recall making this change when setting these devices up the first time around. So, I'm obviously reluctant to make any changes right now to the router settings unless someone from VM confirms this is in fact the right thing to do.

As I've mentioned. One issue has turned into multiple issues now and it's become so much of a headache, I'm using an ordinary, basic clock in the two rooms where I used to (and ideally would still wish to) use my two Google smart displays.

On the plus side. My cam works! But totally cheesed off with the whole saga. Wishing I had never replaced my Hub3 with a red light. It felt stone cold to me and I had no issues.

Sorry this is quite a lengthy reply but I hope this makes some sense to someone and I would love nothing more than to have my two Google displays back working again. I will now read over what everyone else has said but will await further replies before making any changes.

The two devices in question are a Google Nest Display 2.0 and a Lenovo 4" Smart Display with Google Assistant. 

Thanks once again.