Thanks @brada4, though I don't understand using PuTTY any more than any other 'command line' type interface.
I did however, actually 'try' PuTTY (the best I knew how from Linux), entered the default IP with Port 22, and got nothing (no prompts for user/pass), when it went to the next screen.
So far then, I was now able to entirely disconnect the router (except for power and direct ethernet), and accessed LuCI from the default 192.168.1.1. Then updated to the very latest OpenWRT firmware for the WDR3600 (24.10.1 with NO settings saved), and enabled the very basic AP settings that I use it for.
Reconnected to the modem, It now accesses internet from Linux (Manjaro), but not Windows 11, whereas it worked from both, before this all began (out of nowhere.) And still...no LuCI (from Linux or Windows.)
I know nothing, clearly. But if updating to the latest firmware, with NO settings saved (totally clean upgrade), I had sure hoped it would at least put me back to square one, with a working LuCI (just like a fresh/new install replacing OEM?)
Yeah, I know nothing of any CLI-type usage, so unless anyone is willing to help with step 1, 2, 3, etc. with any of those, then I'm simply out of the loop. Maybe, akin to like 99.5% of all device users?
End goal: I'd really like to have LuCI back, so I could at least login to the default IP and do very basic changes over Wi-Fi/browser, as that's the purpose of LuCI even being developed, I'd hoped.
The LAN IPv4 does say the default is 192.168.1.1, so apparently that's correct, but something else must still be wrong for LuCI to be happy, yes? And still, no internet from Windows either, so that's a step back from 'normal.'
Really, very sorry for the trouble...anyone?