Cannot access DMZ (external) when in VPN

[ursus]

I have 4 official IPs. I am using the one IP to run a MailInABox server that does mail, NextCloud, DNS and a web server. I am using a second one to self host an rss server.

My internal network is 192.168.1.x and the DMZ is 192.168.100.x.

I then installed WireGuard on the firewall using port 51820. I am using 10.10.0.x as my VPN network. I setup the network rules and can access the DMZ server and the LAN servers from the VPN.

I then created a port forward to a third official IP that I am using for WireGuard as I would like to use port 53 for the tunnel (none of my clients blocks this port). This just forwards all traffic from the third IP port 53 to the main IP port 51820. This also works perfectly.

This means when I am at a customer I can open the tunnel, ssh to my servers in the LAN and the DMZ.

What does not work is me accessing anything that is on the DMZ side from the tunnel. What I mean by that is that I cannot access eg. Mail on that server. When I ping the mail server I am getting the external IP which is correct, I cannot access any mails using a mail client. I also cannot access the rss server when the tunnel is open... I think I need to set relection somewhere maybe or something else. Does anybody have a pointer as to what I still need to setup?

Thank you very much for any help given!