I can't connect to Minecraft servers with custom Port

To answer your original question: It sounds like netshare is a proxy server. That means that it has to receive the requests as a "server", understand them, and then make requests to the actual server on your behalf. This is easy with common protocols (like http or https), but it very likely does not understand the protocol that Minecraft uses.

Here is another app that might work (disclaimer, I have never used it) It looks like it has a WiFi repeater mode: https://play.google.com/store/apps/details?id=be.mygod.vpnhotspot&hl=en_US

The right answer to your problem is to fix the networking issues. It sounds like you need to run a cable(best option imo for gaming), buy range extenders, or otherwise get a stable connection to your router.