Similar is mentioned in this thread.
What is this “Ims”-thing? It failed with “ims” but there’s no “ims” when it’s working.
Ah, got it: IP Multimedia Subsystem - Wikipedia
Hm, according to this sourcefile the system should try GSM if IMS fails.
And this looks like there’s a reason the old workaround didn’t work anymore, I guess. But this is way beyond my knowledge…