The SIM was the trick.
I set “Use SIM” to off
And suddenly I can see
“System update available”
Download is possible even with SIM turned back on.
edit:
@anon1128606 The SIM which blocks system updates is from Consumer Cellular, MVNO of ATT. Thjs is an American SIM, not a European one, which appears to be blocked.
If you need further testing, I can provide it.