The client behind the 880L bridge running the new beta firmware suffered an unexpected malfunction and became unusable yesterday so had to be replaced, which reset the IP address renewal monitoring by several hours, though as of this afternoon, it looks like the replacement client got one successful IP address lease renewal behind the 880L bridge.
So far so good, and I can continue monitoring that client over the weekend via remote access.
I also rolled back the firmware in my 868L bridge to 1.12B04_WW, to see how it behaves compared to the 1.20B01_BETA firmware. This is a longer term test, as it can take some 2 weeks before a client gets cut off from the internet by a 868L.