Ok, reporting back.
modem_updater -g found “RG501QEUAAR12A07M4G_04.001.04.001”, the old version which would not flash.
The only other version I have is RG501QEUAAR12A08M4G_04.200.04.200 (modem_update_trb500.bin) which still has the speed issues.
So I need either firmware RG501QEUAAR12A09M4G_04.200.04.200 or RG501QEUAAR12A10M4G_04.200.04.200 to fix the speed issues. But neither of these two are available via the updater.
Let me know when you get a solution from support.