Moved from a router 3 to router 5

I've had a router3 for years, it has been working fine on 500mb. Upgraded to 1gig and so have install a new router, since then I have problems all over the place. A gaming PC linked to the router via cat5 keeping saying there is no internet available, My daughter Samsung phone is connected to he wifi and was working, is now saying connected, no internet available, while our iphones are working fine.  When I try to watch apps on my LG TV, it says the connection is unstable and won't run. My nest thermostat is connected to my wifi, but I can't control it via the app. My Eufy doorbell won't connect to it's homebase. Have tried several restarts & reboots.

Beginning to wish, I hadn't upgraded.