I can’t confirm the issue on 1.9-s-20230311268558-dev-FP3 with provider Telekom in Germany, at least it’s not long text or emoji or accent related for me, everything arrives as SMS (according to long-tap - info).
What about “Strip accents”?
You get there from the Message App settings? Are you using the default Message App? In the App, Settings - About Message says Version 3.9.4 for me.
You could still try a different App for testing, e.g. Simple SMS Messenger (com.simplemobiletools.smsmessenger by Tibor Kaputa).
No clue, I only get offered 2G/ 3G/ LTE there.
If you are feeling adventurous while you are in the SIM card settings already, you could try to clear MMS-related “Access point names” (APN) settings, but make a note of the settings before clearing them.
For me there’s an MMSC, an MMS proxy as well as an MMS port given there, and the APN type contains “mms” among other things.