Mac appears to be connected to WiFi but is in fact not

I recently got a new router (upgraded to fibre-optic) and shortly after the installation my Mac stopped connection to the internet (FYI, I have a Time Capsule (with AirPort) connected to the new router). Initially after the change it worked for an hours or so but very slowly so I changed from the AirPort WiFi connection (plugged into the main router) to directly connect to the new router plugged into the wall but then it stopped working altogether. It's a late 2013 Mac Pro running iOS 10.15.4. It says it's connected to the WiFi/router but when I try to connect to any internet site or page it either times out or I get a message saying "You Are Not Connected to the Internet". I have since tried connecting again to the Net via the AirPort router and directly via the router plugged into the wall but neither work and I also noticed that I can't even connect to Internet via the Hotspot on my iPhone. In all tests the Mac says it's connected in the Network Connection menu (Hotspot, AirPort and main router) but I can't access the internet via any of them. I have 2 iPhones, 2 iPads and 2 MacBook Airs and all seem to be connecting to the internet without problems. I've tried to connect on the 'problem' Mac in Safe Mode but that doesn't work and I ran the WiFi Diagnostic option but I'm not tech savvy enough to understand the output file. Any ideas?