@EricSatu 

1 picture tells it all. It's a weird problem.

Can you setup a ACL (Access Control List rule) in the Zyxel2 or maybe in both Zyxels?
Security → Firewall → Access Control

Hi again @yalerta,

I tried but as expected no success. Also ‘ping’ does not work. Definitely an area to explore.

It is a weird problem. Look like something is blocked in the T-Mobile network.

@EricSatu 

In my (our) config there is a tunnel setup with pfSense between 2 TMobileThuis accounts who have both a no-ip domain like your setup. I works flawless for a couple of years now to sync data between 2 servers on both locations. One location has it's own router direct on the fiber, the other has a Zyxel setup in a DMZ setting.
Maybe it has to do with the Zyxel's. The firmware of the router is adapted to TmT so may be crippled.

I suggest to start with some tests with standaard network tools like ping, tracert or make use of some GRC.com or  Nirsoft tools to figure out why?

Thank you @yalerta, I tested more using your advise. Interesting conclusions. My new post will explain a bit, as I follow now a possible Routing issue in the T-Mobile Thuis Network. 

Can’t get my new post uploaded, does the moderator have to screen it ?

Hi @EricSatu ,

You should be able to upload the screenshot without any extra moderation on it, as I don't see it being blocked by our filters. If you have any trouble uploading it, you can send me a private message with a link to an upload location. I thereby can post it here. Please let me know! 

Hi @Boris 

I will do. I had prepared actually another post, based on some guidance from @yalerta 

That revealed more specifics to look for.

For some reason I couldn’t post that yesterday. Will try again.

Title will be different: “Routing problem in T-Mobile Thuis Network ??”

Hope you could check something out based on the new post.