SMS sending broken after eOS 4.0 update (SMSC field won't save) - Galaxy S9+

[AnotherElk]

@YoannC

but the field stays empty after tapping “Update”

Really “Update”? Or “Refresh”?
There are two buttons, and they can be labelled very similarly, not only in English …

@AnotherElk

It’s not very intuitive territory there. This text field can be empty, and the SMSC the phone uses can be fine nonetheless.First use the “REFRESH” button, this will put the SMSC the phone currently uses into the field, or “refresh error” if there is none.
After the phone number of the SMSC there might be an added number given, separated by a comma. This is harmless.
You can check whether it is the correct SMSC by looking it up online for your provider or by asking your provider for the correct number.If you decide to enter a different number here, you will have to confirm your input with the “UPDATE” button next to the field, else it will not be adopted by the phone.