Have you used a mobile phone as a temporary WiFi hot spot, to see if you can onboard everything, as previous, and to confirm all is working, just as it used to? Would be an annoying coincidence, if for example the sync module had failed, and all the while you were trying to diagnose issues with the new service provider and router.
If that works, next thing I would do is log into the router, and temporarily turn off 5Ghz. If there is any fancy auto switching going on, then it can’t with 5GHz switched off. If it then works, assess if you’re happy to manage with 2.4GHz only, and if not, see if you can find a specific setting, regarding smart switching or similar, and disable that.
Sorry I can’t give specific advice about the provider, but I’ve never heard of them. If none of it helps, I’d give support a call. At least if there is a specific issue with that provider and router, support will likely have already got to the bottom of it.