Sorry joco, but that's not easyly done for the quickest protocols ( > 2400bps).
I think (I'm not sure now, need to review some old stuff) that the
slowest protocols (300-2400bps) were half-duplex. In this case, you could
do the job but it is so slow and still a pain in the ass to implement (you need to simulate ringing tones, etc). There are much simpler ways to connect 2 computers via a radio link. The easiest is to attach an FM transceiver to the serial port: you get nice 115200 bps BPSK modulated with almost no effort. If you want much more throughput, you can use the parallel port but then you must serialize the output of the port and use previos method or use a 8PSK digital transceiver. Just one point, you should check your local law because you can be using more bandwith than allowed with such methods.
Now, the reason why modern modem protocols can't be directly radio linked. High-end protocols are, mainly, simetrical full-duplex. This means some bandwith is reserved for the uplink and some for the downlink. The problem is that what is cleanly separated at low-frequency it is not in
high frequency and then both channels interfere each other. Other considerations are the inconvenience of a MQAM modulation used in this protocols. It is not a good idea to use such in a radio-link without using any corrective methods.
Summarising, it is not feasible to connect 2 modems at high data rate
via a radio-link by just connecting them a a RF transceiver.
By the way, why not using a standard solution like WiFi or WiFi5? Do you have line-of-sight path between your computers? How far are them from each other?