I'm not setup to cross compile code to target OpenWRT. And I would probably have to know the exact version of the router to make it work, as OpenWRT runs on a variety of things.
As an alternative, I'd suggest getting a RaspPi Zero 2W. For $15 you get a quad core 64 bit ARM running at 1Ghz with 512MB of RAM, and WiFi connectivity. And it runs NetDrive ... Netdrive is only consuming 6MB of memory, leaving 190MB free for other uses. Add a small case, re-use a power supply, and add a small SD card and you are set to go. As a plus, you can run other servers on it like Telnet, so you can bounce from mTCP Telnet to the RaspPi, and then out using SSH for systems that require that. Or run Links (a text mode browser), or do many other things ...
A DOS virtual machine on my giant Xeon system talking to NetDrive on the Pi Zero 2W is recording 400KB/sec for reads and writes. That is as good as real hardware (a 386) talking to a Netdrive server on the same Xeon box.
-Mike