Serial output from a Belkin F5L049

I am currently poking about with a (so far) unmodified Belkin Home Base with a vague hope of maybe someday running OpenWRT on it. My first target is getting a serial connection. There's a 6-pin JST connector on the PCB, which I'd assume to be the what I'm looking for. I've identified ground and 3.3V pins, but the other four are bit of a mystery. Two of them are pulled up to 3,3V, the others, seemingly, to ground. Hooking them up to RX of a USB to serial adapter (actually a Chinese Arduino Uno clone with reset tied low, but hey, it has worked so far) has yielded no results in PuTTY (tried various common baudrates), but one of them does make the RX LED on the adapter blink in a seemingly non-random fashion.
Any opinion on what could be happening here? Is the supposed serial port not a serial port at all? Could the voltage mismatch be at fault? (5V systems usually recognise 3,3V as high, and I'm not connecting 5V TX). Is there any hope I can solve this without busting out a logic analyzer or an oscilloscope? Any insight is welcome, including any good reason why this could be hopeless (what with AR9xxx seemingly not even on the list of build targets)

While it should technically be supportable, chances that this device ends up on the desk of a developer who's also motivated to work on it (for something between a long weekend and several weeks) are somewhere between very low and non-existent. If you're interested in this device, you'll probably have to port it to ath79/ master yourself.

Yes, the device is supportable, but it's eleven years old by now. If it didn't attract anyone to work on it within the last decade, it likely never will - it's low-end, buggy draft-n wireless silicon, single band, slow CPU - you can get much better stuff for under 10 EUR/ USD on the used market (or starting under 20 EUR/ USD brand new, delivered to your doorstep).

Pardon necromancing a topic, just thought I'd drop the few proceedings I've had in these last weeks for any other poor soul with too much time and a similar router on their hands.

Continued tort...,er, interrogation of the presumed serial port has been, so far, fruitless. The next point of interest was the original firmware, which I tried to scan with binwalk. That gave me the the very informative output of nothing - quite literally, binwalk printed the top row of the output table and nothing more. Something wrong with my Lin ux installation, or is the FW just that obscure?

Doing a bit of forum archaeology, I stumbled upon a post from the last year, which mentioned using the Linksys WRT160NL build for AR9130 based devices. Not sure how much point is there in prodding further, but I suppose there are worse ways to waste one's time.

@slh, don't think I've ever seen an OpenWRT-supported, 16/64 device with 4-port USB hub going for 20 EUR on the used market, let alone new. Care to elaborate?