@Gepepe incompatible routers doesn't mean anything, it's not how it works.
This just means that whoever is hosting is refusing the connection: UPnP not working, bad port forward rule in the router, firewall on PC blocking, your antivirus has a firewall blocking etc.
Port forward on the router instead, only the host has to do this and the client UPnP doesn't matter.
https://plutonium.pw/docs/custom-games/#what-is-upnp
https://plutonium.pw/docs/custom-games/#if-upnp-is-still-disabled-or-you-cant-connect-despite-it-being-enabled
Also make sure that you're in-game playing with a pistol when being join and that you're both on latest version (r3043 at this day) https://plutonium.pw/docs/update/
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