AR300M-ext looking at front of router which antenna port is receive , if I wanted to use an outside panel antenna for greater siginal.

With AR300M-ext it is possible to exchange the external antenna for another external antenna.

I see you have limited knowledge of this router I know and instant well antennas can be replaced on This router as it has two , questions are which side of router antenna connection for receive?

Booth AR300M-ext antennas are used for sending and receiving.

In most WiFi antenna systems one antenna is the ‘main’ antenna and the other is ‘RX Diversity’ which allows better reception in multipath situations. Only one antenna will be viable for extending range, the other will be essentially useless if deployed significantly away from the first one - such as would be if you put one antenna outside and one antenna inside.