Linksys E2500 Software limited MT76x8 routers, Image signing

Linksys currently has a router series that by report, are all the same internally. On the low end, the E2500 is limited to N600. The same report says it has AC1200 hardware capability.

I can imagine that Linksys must have some mechanism to prevent someone from buying the cheapest hardware variant and flashing it with firmware from a more expensive version.
In this E2500 v4 thread, the firmware installation had to work around a signed image.

Anyone aware of a work-around for Linksys image signing?

From this post, it looks like you could do it via serial: