Mesh WiFi has always caused problems for anyone using them.
The specs for your devices are:
Telekom Speed Home WIFI Solo
Mesh WiFi Repeater.
Wi-Fi 5 (802.11ac) up to 1733 Mbit/s (2.4 GHz, 5.0 GHz)
2x GB-LAN.
All-IP ready.
In my apartment, my internet connector is in my bedroom.
All my equipment is in my office.
Wireless for the main internet service doesn’t work well for running a server.
I ran flat ethernet cable (it looks like a ribbon) under the edge of the carpet, along the wall, from my bedroom - down the hallway - into my office and around to my server rack.
I don’t know how to advise you here.
I know that Plex isn’t causing your internet disruption - UNLESS - it’s using all your bandwidth for streaming.
It is VERY sensitive to disruptions.
I noticed you have Remote Access enabled.
If you disable Remote Access and then play something with PMS,
does your internet still get disrupted?
i cannot answer that yet i will need to try that but the remote access is also kinda weird because i was outside on a computer wanted to watch a movie and it doesnt work because it didnt had a secure connection?