FritzBox with external Antenna (restart necessary multiple times / day)

Hello forum-members,
finally I do have the time to describe my problem with the OpenWrt software and an external antenna, connected to the USB Port of a FritzBox 4040 router.

I should mention, that I use the external antenna to connect to the router of my parents house, while staying myself in flat across the street, the distance is approximalety less than 20mtrs. The sight for the external antenna is free and directly pointed (with maximum signal strength) to the router (also an FritzBox 7490)

So my setup is as follows:

connection to internet
FritzBox router 7490 (my parents house)
connection via external antenna (USB Port)
FritzBox router 4040 (me)

I was using OpenWrt (must have been version 20.x or sth close to it) and the connection between the two routers was really steady. No problems back then. After upgrading to version 22.x, the terminations of the connection began. Sometimes once a day, sometimes multiple times a day.
Nowadays at most times it terminates the connection to the other (Fritz7490) router when I log in the wireless network, provided by the router (Fritz4040) in my flat.
The wireless chip for the external antenna is labeled as MAC80211 802.11b/g/n (in OpenWrt). The driver I use for the external antenna are from Ralink Drivers for RT2x00 cards (rt2800 LIB). In addition for the usb connection I use Ralink RT28xx/3xxx USB firmware and Ralink Drivers for RT2x00 cards (RT2870 USB).
The hardware of the antenna is labeled as ID 148f:3070 Ralink Technology, Corp. RT2870/RT3070 Wireless Adapter

The other two wireless radios, built in the router Fritz4040 for my home wireless network are labeled as Qualcomm Atheros IPQ4019, for 2,4 and 5Ghz.

What else do you need to dive into a solution finding process?
I suppose I should post some ot the logs, but which ones are interesting?

Looking forward to read from some of you experts!
Peter