I have a WRT54G v2 with the threaded portion of the left side (viewed from the front) RP-TNC connector pulled off from the the rest of the connector on the PCB.  This is the antenna port with the cable attached to the PCB.  I've cut the cable from what was left of the board-mounted connector and desoldered the cable from the PCB, then soldered a 47 ohm 1/2W resistor on the PCB as a dummy load.

I connected a 41' piece of LMR400 on the right (good) antenna port of the router with a RP-TNC to N adapter, and ran the cable outside to a 12dBi panel antenna.  I can change the TX/RX Antenna setting to have both going to either Right or Left antenna port, but the signal strength is the same when measured with a WiFi utility on a laptop near the antenna.

The "How to use the web interface" in the User Documentation says that "viewed from the front" is correct for the GL series but may be different for others.

How do I verify I have the proper antenna port selected?