X750 Wireguard extreme slowly

I have an X750 (v2) with Firmware 4.2.2 that I use in a camper van. At home I have a Fritzbox 5590 Fiber, on which a Wireguard server runs, which does its job really well. Now I have set up the client on my X750, but it is extremely slow and I noticed that it is very slow at all, regardless of whether it is in repeater mode or if a SIM card is inserted! I took some screenshots for a better explanation, can someone help me here, the router is almost useless because I also use it to do TV streaming!

Looks like a MTU issue for me.
Send from Android over Beryl AX to internet → good
Send from Android over FritzBox (WG) to Internet → good
Send from Android over Beryl AX (WG over FritzBox) to internet → bad

So it seems the packets send within the VPN are fragmented and the solution is to reduce the MTU (Maximum Transfer Unit). Or switch to IPv6.

Before research how to adjust MTU at every device, I would use a Laptop and try different packetsizes by ping with payload and set the ‘no fragment’ flag, to confirm my theory.

That’s really nice, but I have no idea about it! But actually it is like this:

Android- Hotspot (wifi) -very good
Android- Spitz (wifi 5ghz with Repeater Mode) -bad
Android- Spitz with Wiregard activ ( also Repeater Mode) -more bad
Android- Spitz (SIM Card Mode) -more bad

Today I reinstalled the old firmware 3.216and now the modem (SIM) works again, but Wireguard cannot be activated in modem mode. It shows “active” (green light), but I tested it and there is definitely no active tunnel! But it works in repeater mode???

does nobody have a solution?