Thanks Poster!
I don't use another router and I have no problems connecting and using Musicbrainz.org in my current configuration.
Is this MTU clamping also used to connect to Discog.com? Or any specialized ports?
If I make a VPN-connection (using Windscribe) I can connect to Discog.com without problem. So I think it is my internet-provider.
I would like to know what to ask them. I mean: what port or network traffic is probably blocked?
As a test, I added a screenshot of the NETSH-command suggested in the "Can not reach musicbrainz.org with WSS)" you suggested. Maybe this gives another insight.
