When i upgraded from 350 to 500 and doubled to 1gig my speeds were also rubbish on the hub 4 using a Hub 4, the engineer swapped it out for a hub 5 and my issues were sorted but i dont have it in modem mode.
Hi there @keithsb, welcome back to our forum and thanks for your post.
I'm sorry to hear you are not getting the expected speeds since upgrading your services. I would recommend first running a fault test via our service page here to see if this helps at all. If you have no luck with this then it would also be beneficial to go through some checks here to help boost the speeds.
It would also be best to pop the hub into router mode to see if you see the same issues.
Let us know how you get on with this, if you need any further help we'll be happy to assist.