When helping a colleague at work who was struggling to get the correct IOS image onto a ISR4431 I came across this tip that I never knew, in later versions of ROMMON you can boot from USB.
All you need to do is:
1) Ensure router is powered off
2) Insert USB stick with IOS image on
3) Boot router into ROMMON
4) Type the command: boot usbflash0:
Obviously the “usbflash0” may change dependant on platform and which slot you plugged the stick into.
This is really helpful if the IOS is corrupted or missing.