Internet crashes with high load, e.g. Streaming etc. From?

Moin since I found no help on websites, YouTube and other platforms

I now want to ask whether you have a solution to my everyday problem.

So: I bought a new tariff with Vodafone some time ago, i.e. 25 Mbit / s upload and 250 Mbit / s download. I used to have a 2 Mbit / s upload and a 30 Mbit / s download. I have a good LAN connection to my PC (LAN connection) and wanted to stream the new tariff on Twitch (now I stream on YouTube) and it was faster than before in the game I stream (Minecraft on servers). The only problem is that now, even with green in OBS, my internet always crashes after a certain time. It is different, sometimes at the start of the stream, somewhere random. Later I noticed that even if I do something other than stream, e.g. Talk in Discord, then I still have Minecraft open, and e.g. Open a program (in which case I wanted to play Valorant and it made a download) the internet crashes. We've tried everything, YouTube, searched somewhere in forums, and even the technician came over and said we finally have 4 upstream channels (even if I don't understand what he means) the internet was still so bad. In the past with the old tariff I could stream in Minecraft with 200 ping but for several hours without the internet crashing. It's getting annoying and I hope you can help me. My parents think it's because of the PC. The error message in OBS is: Connection to server lost.

Do you have a new router since the new tariff? And does the internet fail on all clients in the network or only on your PC?

So we normally have a Fritz Box and a router from Vodafone to which the Fritz Box is connected. And so far the internet is only down for me

Why do you have a Vodafone router AND a Fritz! Box?

We used to have that too. Only with another Vodafone router because with Fritzbox you can control everything better and the Vodafone router gives the Internet as far as I know, but I don't know it very well now

So you can only use the Fritz! Use the box, which is a full-fledged router. Your internet may be more stable with it.