How do I reset a dhi-nvr4104-p-4ks2?

[glisignoli]

I've got a second hand dhi-nvr4104-p-4ks2 but I am unable to reset it.

There's no reset button and I can't see a place to wire in a serial cable

 

[alastairstevenson]

I can't see a place to wire in a serial cable

If it was mine, I'd check out J29, above the board part number.
Serial TTL, not RS-232