Exactly the same issue here. I thought I am just stupid.
Login to zwift on website or in the Desktop App is not working during high usage hours (like now)
Login to Companion App same, not working via cabled Internet from Deutsche Telekom.
Using a VPN into the company which is using another Provider is working fine. Mobile Internet connection is working fine on my Smartphone.
It seems to be working (even slowly and with disconnects) when not so many zwifters are online.
I am using Hybrid Internet via 4G and VDSL, but no matter I am using only the DSL or the 4G, it is not working.
I tried lots of other DNS Servers, no change.
Internet itself is working like a charm, couldnt be better.