And now a screenshot of "ipconfig /all" in Command Prompt.

Your primary DNS-server should be the same ip address like your default gateway.

Done. The Default Gateway's IP is the same as the IP of the DNS Servers. I manually had to assign it. Restarted my router after this. But the diagnostics are the exact same.

Restart the router again and run the game diagnostics. Then check, whether your router's firmware is still up to date.

All done. No change in the diagnostics.

Yes. Did that as well. It is upto date.

In an earlier post I suggested resetting the Windows Defender Firewall. Have you tried that as well?
If yes, we have properly configured everything. This should work now. :/

Uhm wait, in the most recent IP Logs you are showing an IPv6 address again...
Please make sure, you use the IPv4 on your WAN address.

I've completely disabled IPv6 from the configuration. But it still does not work. And yes I've configured the setting on the windows defender firewall as you said. But the results don't change.

The DHCP server also uses IPv4, not IPv6.

Is this somehow an issue of DYNAMIC IP setting enabled?

I am talking about the IPv6 address on your WAN, not the IPv6 support within your network. You have two IP addresses, an IPv4 and an IPv6 address, but apparently it keeps using the IPv6 address. You however require an IPv4 address in order to play those old DPlay games. Try calling your ISP!

Alright. Thanks a ton for the help above.

You already have both IP addresses assigned; just call your ISP and have them removing the IPv6 or ask them how to disable the IPv6 address. Some routers allow this, some routers don't and the ISP must do this for you.