Disconnected from BNET on WAN1 but able to connect on WAN2

So, something weird, I recently noticed (and I already opened a support ticket for it) that on my residential ISP (just a normal provider, no business contracts) that I was unable to connect to battle.net, it gave me a ‘failed to connect to battle.net’ after verifying account.

I opened a ticket and they told me to uninstall any VPN software, any antivirus software just for the sake of it, which I did.
I waited 2 weeks (or more) before trying to connect again and the problem remained.

I have a second WAN at home with a different provider (residential aswell) and I though that it would be a good idea to test with that line.
with the same laptop I tried connecting to diablo 2 and was able to connect without any issue… so clearly it is not my laptop providing me this hassle but rather something that blocks my main ISP?

I would really like this so be cleared out, it is not the intention to use my backup WAN which is a very small bandwith line, to play Diablo 2.

please advise?