U.S. Robotics serial external modems were the best available in my country. Using RS232 ports, they (mostly) do not need any driver. ISA modems should work, too.

Just avoid any USB or PCI modem.

Are there no DOS compatible modems for PCI? Also, what would you recommend for ISA? Is does an ISA modem have any latency advantages over an external serial one?

Most PCI cards were winmodem and those modems decoded signals using your CPU. That lead to high CPU usage and unstable connections (if your CPU did not decode the signal in time, the connection dropped), so even in Windows they were the worst option. I heard of some PCI cards that had proper chipsets that did everything without CPU intervention, but I have never seen one of those.

Most USB modems are winmodems too, using the USB connection to transmit undecoded signal. So you must add the USB latency and CPU usage to the disadvantages of PCI winmodems. Even if you can't use them in DOS, don't buy a winmodem or USB modem if you intend to use it on Linux or any other OS that is not Windows... even if you find drivers, the connection will drop frequently.

Also, there were a few ISA winmodems (as rare as PCI non winmodem cards). Modems are slow sending data, so I don't think bus latency will be an issue. Keep in mind that ISA and serial modems had top speeds from 1200 bauds to 56K, so you'll have to be sure that you are buying a proper 56K device. When 56K standards were "not standard", some brands launched 33.6K modems that could be upgradable (via firmware) to 56K standard. Don't rely on those modems... maybe those brands have dissapeared and you can not update your modem.

On second thought, 56K standards were launched at a time that ISA cards were becoming rarer. Maybe you won't be able to find a 56K ISA modem. Also, 56K standard means that (depending on your analog line) you may get up to 56K of download, but upload is limited to 33.6K. Games that used serial connection will not connect at 56K (because the uplink is limited), so maybe a 28.8K modem will be enough for your purposes.

TL;DR: Buy a serial modem with a big 56K, 33.6K or 28K printed on the case.

You don't use a modem for a "null modem" cable, it's just a serial crossover cable

If its 2 PC's in different locations and you want to phone the other PC, you want an external modem! if nothing else then for the pretty flashing lights 😉 (But seriously, it'll help with troubleshooting as well)

If its 2 PC's in the same location as Matth79 said, its just a cross over cable.
In both cases you will also want a high speed or 16550 UART serial port, Otherwise the serial port will bottleneck you anyway.

There definitely were 56k modems for ISA and for RS232 - I had both, back in the day, and I still have the serial ones kicking around in my spare parts locker. They might even still work. There are some actual non-Winmodem PCI modems, but they're pretty difficult to find. Manufacturers at the time went to great lengths to obfuscate whether their cards had controllers on them or not. As for finding one, look up resources about using modems under Linux that date back to the right era (1997-2003 or so). As I recall, Eric S. Raymond wrote some Linux workstation builder guides in about that time period that addressed the issue, and I believe some specific models of PCI modems that do work were identified. The usual advice, of course, was "an external RS232 modem is always a real modem; use one if you can."