Having here (Pixel 6a) problems with sending sms… (receiving is ok).
Where is the Short Message Service Centre (SMSC)?
I can’t find it in iodeOS, is there no entry for setting up?
Correct, I don’t think it is (easily) configurable.
So receiving is fine? It isn’t attempting to migrate it to MMS formatting is it maybe?
How can i detect it?
Yes receiving is fine, all time.
I will probably try to insert the SIM card into a mainstream Android and see what happens, if there are no other solutions / explanations…
My experience is now quite outdated but when on Ubuntu Touch in approx. 2021 (since VoLTE became required in the USA I had to leave UT behind sadly, even though there is now recently some progress in that area…), I am pretty sure I had an identical issue. The problem ended up being at the carrier level, where effectively they were detecting that my device hardware was capable of RCS messaging, so they were essentially “auto-upgrading” my SMS to RCS. I can’t recall if they (carrier) were able to disable something, or if some change was made in the UT port to basically disable reporting the device was RCS capable.
Sorry my memory on that issue is a bit hazy, it wasn’t on iodéOS, it was with a USA carrier (T-Mobile compatible MVNO), etc. so it may not be relevant at all. But I thought I did remember that RCS was involved. Is this a new number for you? Maybe you could check if RCS is disabled on it to make sure? https://messages.google.com/disable-chat
Now possibly the issue was on receiving plain SMS (even though MMS continued working), but I thought it was in sending?
You can get to this after dialing *#*#INFO#*#*, but it only includes “Update” and “Refresh” options.
I will try and report, thank you.
Thanks, I keep forgetting what 4636 stands for 
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Consider trying a different carrier. I was using AT&T (in the US) for years and constantly dealing with unreliable MMS, voicemail, wi-fi calling, and even 5G labeling on LTE service. The tech support agent at US Mobile told me that AT&T is not friendly with older phones or de-googled devices. So I switched to Verizon service and all those issues disappeared instantly.