Hi Op -my partner who works in IT troubleshooting has suggested the following......
Assuming it was fine before your upgrade and you get the same bad performance using Wired (Ethernet cable) or wireless connection. Try any or all of the following suggestions.
Apologies if any of the following has already been done....
Try changing your broadband filter (the one that the router and house phones plug into.
When the router is plugged into the test socket make sure all the other house phones etc are disconnected. This Will rule out a problem with the phone wiring within your house which is your responsibility (cost). The test socket and outside wires are your internet providers responsibility .
Is the "drop off" a total loss of internet as indicated on your router (it will have lights on it to tell you the status, power, Line and connection)?
If so, then you have either a wiring fault somewhere, or a router fault.
if it is just a slowdown of the broadband speed that is the problem you can try doing a Broadband speed test at various times of the day and noting when it is fast and when it is slow. If there are patterns in the speed changes (teatime always poor) then it is probably a bandwidth issue.
Try using the internet on another device, i.e. phone, tablet or TV. If these work OK then check your Computer with your anti virus software and a reputable anti malware program i.e. Malware bytes anti malware (please be careful when running the setup programs and read the screens and make sure the ticks for the trial version or any additional software are NOT ticked).
As all the providers use the same physical wires a change to another provider can be just as bad. As you have the router in the test socket and it looks like they have reset your line it could be a bad connection on any part of the wires back to the cabinet or even exchange.
This type of fault usually takes them a while to find because BT own the wires and it will cost your provider money to get it checked and fixed. Oddly enough it is the last thing they will check. Have you had some storms lately? These can cause physical damage to cables and connections.
Or it could be just like the other PP suggested that your provider has not got the capacity to give all the users in your area fast BB all the time.
Hope this is of some assistance.