These ones failed to boot:
v3.0-r43516 std (06/25/20)
v3.0-r44700 std (10/30/20)
v3.0-r44715 std (11/03/20)
v3.0-r45219 std (12/31/20)
v3.0-r51576 std (02/04/23)
I would like to install Micro RedBoot. The Fonera 2100A is AR51 design, has 16MB of RAM and 8MB of flash. So prefix RedBoot_AP51_16M_8M_??? looks like a good candidate.
But there are four switch variants to download: Kendin (Realtek), ADMtek, Micrel, Marvell
I faced the radio failure with some builds as well.
I'm pretty sure there was no problem when flashing which took very long as you stated. I had enabled keepalives to prevent the telnet session into RedBoot from timing out.
Before I do much more testing I will add a serial console to be able to check messages when booting.
However I would like to install the Micro RedBoot also and don't know which one prefixed with RedBoot_AP51_16M_8M_??? to choose. Since I don't want to brick the device I would prefer to use the correct image.