I had the same problem as yours while upgrading, albeit on a hAP ax lite LTE6. However, both share the same LTE modem, FG621-EA. Here’s how you can upgrade yours manually to 16121.1034.00.01.01.04:
1. Download the firmware patchfile.
/tool fetch https://upgrade.mikrotik.com/firmware/FG621-EA/16121.1034.00.01.01.03/image
2. Upgrade manually.
/interface/lte firmware-upgrade lte1 upgrade=yes firmware-file=image
3. Delete the patchfile after the successful upgrade.
/file remove image
You’re welcome. Glad I could help. You might want to check out v7.12rc4 and later, which include this fix:
What’s new in 7.12rc4 (2023-Oct-27 11:51):
*) lte - fixed FG621-EA possible timeouts during firmware upgrade;
It might have fixed modem firmware upgrading, but that remains to be seen until a new one is released. Until then, manual upgrade as specified above is the sure way to get and install the update.