I have the exact same problem with the BE65-5G router as decribed here: https://community.tp-link.com/en/home/forum/topic/736386 – it keeps turning red and losing the network. The only fix is to set the BE65-5G to "4G only." External antennas are connected, and the signal is excellent....
 

I also think that the information provided by German 5G providers is sometimes outdated or incorrect. For example, O2 is still listed as E-Plus, and you have to enter the APN settings manually.

This is probably also the reason for the occasionally very unstable internet connection with noticeable interruptions when switching between cells — despite good connection parameters (RSRP: -75 to -85 dBm / RSRQ: -6 dB to -7 dB / SNR: +9 dB to +22 dB).

In 5G mode, the BG65-5G regularly loses connection completely during cell switches and shows a red light. In 4G mode, at least the connection is maintained (with a short interruption) during the rare cell switches without needing a restart.

For such an expensive device, I find this very disappointing, especially since everything else about this router is almost perfect.

...I hope that this will be fixed with an upcoming firmware update soon...

Jack