Internet problems on your cell phone?

Hello,

Since I got my new phone (S21+), I've been having internet problems. My Wi-Fi simply disappears in my room, and I have to hold my phone in what feels like 10 different places to get a connection again, which disappears after a few minutes or even seconds. I've already reset the router, removed and re-added my phone, changed the router name, and tried everything else. On my old phone, it worked flawlessly.

Any ideas to fix the problem??