I've released Heddle, a simple way to connect retro PCs to a network over a null-modem serial cable. It is especially suited to running on an OpenWrt router like a Gl iNet MT300N, but is cross-platform and also runs on Windows, Linux, macOS, and Raspberry Pi.

It supports:

- Online and local IPX networking for DOS games
- SLIP networking for DOS and mTCP
- PPP internet and LAN access for Windows 9x
- Hayes modem emulation for modem games and telnet BBSes
- Multiple retro machines from one host
- A web interface for setup and monitoring

Heddle is distributed as a self-contained program and is intended to be straightforward to install and use.

More information, downloads, and setup guides:

https://github.com/RetroLoom/heddle