I had issues with a Linksys EA7300 v2 that arose when I switched from a DSL/1port router to a cable modem. I watched the front panel blink through 3 cycles of booting than fall back to the Linksys image. I found a post that some cable modems only provide a DHCP address on startup so I powered on the router and modem at the same time and the router booted right into OpenWRT. I'm suspicious that Linksys MT7621 devices running OpenWRT will fail to boot if a DHCP address is not available to the WAN port (have not tested to see if jack sensing may play a role).
I also enable the rc.local env commands but noted that the rc.local on my 21.02.1 release was not executable. ls -al /etc/rc.local
to check.