I did upgrade the firmware with QFirehose successfully while modem was attached to the router (but not in use). Just unpack the firmware zip file into a subdirectory and run QFirehose like this:
QFirehose -f EC25EFAR06A21M4G
where EC25EFAR06A21M4G is the subdirectory that contains the firmware.
You’ll need to have QFirehose compiled for the host architecture. The source code is at Github.
There is a video tutorial at How to upgrade module thru Linux QFirehose