Netgear WNDR3700v4 not booting anymore after upgrading to 21.02.01

It is a Netgear router, so it likely has the same TFTP recovery flash routine in the bootloader as e.g. WNDR3700v1 and R7800 etc. You should be able to recover the router by flashing a new "factory" image with that TFTP tool.

(WNDR3700 v4 is different than those routers I mentioned, but Netgear has been pretty consistent with the bootloader)

See advice e.g. in