The router I'm working on is a Telsey CPA-ZNTE60FWB, openwrt works great on it (probably a branded variant of CPA-ZNTE60T already present in the wiki) but I need to add an external antenna to use it in my environment.

The router uses three antennas soldered on the PCB, they are simple piece of metal between antenna output and the gnd. I tried substituting one of them with a connector to my external cantenna without success, if I connect or disconnect the external antenna nothing changes.

The first differences between the two kind of antenna is that the one on pcb is "shorted" to the ground but my cantenna doesn't.

Taking a better look to the pcb I've noticed empty placements that imho can be eventually used for external antenna connectors, aren't they? (MJ2 on the right side of the picture)

I think changing MC571 and MC572 I can "redirect" the signal to an external connector, what's your opinion?
Any suggestion on what I can try?

i would give this a try.

I tried without success but investigating on the chip RT2529 I've found it implements MIMO.

So I think it should be the problem... accordingly with its architecture block it should uses different antennas to transmit and to receive, shouldn't it?

IIRC some drivers allow to switch between auto select or using one particular antenna. See also http://wiki.openwrt.org/doc/uci/wireless

I already tried disabling diversity and using the same antenna for both rx and tx... but I haven't found a way to establish which antenna is in use so I tried empirically with 0, 1 and 2 without success.