Wifi shouldn't do much other than a LAN does, except when the whole connection is spoiled. But since you can connect to it from another machine from your network, that seems not to be the case.
1. Does your router have "Utilities" on it? Like a ping utility? If so, can you ping the machine the mt server runs on? What does it say?
2. If you have xampp on the machine the MT server runs on (or any other service that you can try to connect to): Run it and try to connect to it the same way you would connect to your MT server. If this works, the problem is in your MT config, not in your network. If it doesn't, the problem is elsewhere.
3. Is your DMZ mapped to an IP? And is that IP the same still? DHCP at least on my router tried to assign the same IP to the same machines, I don't know if your router does so as well. What happens, when you try to ping your WAN IP from the outside? Make sure, the machien in question still is the host the DMZ points to.
4. What exactly happens, when you try to connect from the outside? Does it say "timeout" or "no route to host" or something else?
5. I'm not sure how it is on your router, but on mine I can either have a portforwarding OR i can have a DMZ. Try disabling DMZ and experiment with port forwarding OR use the DMZ and don't count on portforwarding
6. Could it be that your ISP doesn't forward incoming connections to you? Or you are behind a proxy of some sorts?
7. Not sure if it's a good idea, but you may want to disable IP v6 for the sake of the experiment
8. What does the debug.txt or the console of your minetest server say? And what does netstat say? Which port and which IP address does the server listen to? Is it really bound both to IP v6 and IP v4?
9. Can you share your screen with us, like on twitch or something? If so, make sure nothing personal or funny is for us to view. Same goes for your WAN IP. You blacked it out, so I assume you don't want to share it with all the world.