If you open the AR750s, the antennas should be connected to the pcb with U.FL connectors. I haven’t seen the AR750s board but i think one antenna is for 5ghz, the other 2.4ghz, so you could just unplug the antennas and use any antenna you like.
There are a lot of different antenna types, so it’s up to you which you replace with.
Antennas can have a lot of gain with a shallow beam angle, or omnidirectional, depends what you will use it for.
Just look for wifi antennas, 2.4ghz and 5ghz, that have the requirements you need, and U.FL connectors.
EDIT yes its U.FL: