Own Minecraft server only works through Gateway! As soon as I want to go through the router, it will not work anymore! Why?

I put together my own Minecraft server via computer. Unfortunately, I have the following problem: We have two devices for the Internet. Once the gateway and then the router TP Link Archer C1200. If I now just connect to the gateway and unplug the wireless router, the Minecraft server works without problems and I can gamble with my uncle. But I do all this now via the wireless router, my uncle can't join! Have in the TP-Link router really tried everything for the purpose of port sharing, etc… Just does not work! Does somebody has any idea? I would even be willing to buy a new router that will work!

What exactly do you understand by "Gateway"?

A gateway is usually the IP address that connects the PC to the Internet, usually that of the router.

How is this "gateway" wired?

By gateway I mean the modem. So modem and router separately. Can this be the problem?

Then you have probably made the port release in the router is not reasonable. What is sent to the Internet depends on the device that sends to the Internet. The only thing that the router prevents to my knowledge is the unauthorized receiving of connections on unreleased ports.