In Nov. 2022 I opened the following thread: https://forum.openwrt.org/t/c2600-wifi-sta-mode-works-with-5g-but-not-with-2g/141956?u=barney
But my success message was too early. 
Based on some recommendations in this forum I bought an Allnet ALL-WA1200AC USB-WLAN-adapter. It has in-kernel driver support and works flawlessly (specially in sta mode).
By pure chance I detected, that the C2600 built-in 2G radio is able to connect to an AP in sta mode, when the Allnet device is plugged in an USB port and not active. When unplugging the Allnet device from the USB port, the connection between the C2600 2G radio and the AP is interrupted. Inserting the Allnet device again, the connection between the C2600 2G radio and the AP is re-established. I can reproduce this behaviour on demand.
I find it rather weird, that the success/failure of a WLAN connection depends on the pure presence of another device.
I'm far away from being a WLAN expert and have no explanation for this behaviour. Perhaps somebody else? In the logs I can see just the success or the failure of the connection attempt.