Locked out of WRT 1900AC V1 with Davidc502 Mamba

So here's the history, the above-referenced router with Linksys stock was disabled by power surge and sat unresolved for a couple years after failed ressurrection attempts with both stock firmware and DDWRT until I stumbled on OpenWRT just this year.
Easy and flawless flash using Davidc502 Mamba build, but similar connectivity issues as the upgraded stock firmware, went unresolved, despite various tests (February 2019).
Yesterday two ISP techs came to fix home cabling at the fiber node, and I bribed them into installing this router for me.
(I live in Asia and our respective communication skills did not allow for much technical talk while they worked)
It works perfectly, but now I can't log in using the ssh/terminal or LuCi.

So I am at the point where, yes, I have figured out the failsafe workaround, but catch 22, cannot log into it.

Could the techs have a tool that would have logged in, and then changed the router password?

When I first invoked ssh, after the techs left, I got the checksum 'warning' response, so how'd the techs bypass that?

Besides that, I did not then see the welcome screen, it immediately asked for a password.

I'm almost certain the password I have is correct. The browser even stored it twice. At one point back in February, I accessed LuCi on root192.168.1.2 (or same password record nevertheless.

So is it normal to get the checksum warning only when logging into ssh, even if I used LuCi successfully prior to that?


Apart from a 'burn it all down' reset. Is there anything else I can do to get back in?

Is it possible that they reset OpenWrt to default config, where password is black?

Also are you sure you have the correct IP and you are actually trying to access the router not another device on the network?