Attach router's WAN port to internet (or at least a DHCP server).
Connect to router's wifi (it has no password per default) then go to http://192.168.31.1/ and click through the setup wizard.
When it asks for a password just give it something easy to remember.
Once the setup wizard is done, connect your computer to one of the routers WAN ports and again go to http://192.168.31.1/ and enter the password you set to log in.
I recommend flashing the version 2.0.23 firmware before going any further. I know the exploit works on some newer firmwares but I also know it doesn't work on some 1.x firmwares. Search the web for the filename firmware miwifi_rm2100_all_fb720_2.0.23.bin to find the correct firmware.
Flash the firmware. The flashing area in the web gui is accessed by clicking the middle of the three groupings of chinese characters at the very top, then clicking orange/yellow circle on the right with the "i". There will be a button that opens a File browser toward the top. Select the firmware file and click the obvious button to flash.
Flashing will take a couple of minutes. Once it boots back up, again go to http://192.168.31.1/ and log in, then follow Installation Method A from here.
If the javascript exploit doesn't work, try the shell script here.
Once you've installed OpenWRT remember to immediately install wireguard or any other stuff that requires kernel modules since you otherwise will have to do a sysupgrade if you want to install kernel modules later (because this is running snapshot and they change often).
Also note that luci is not included in snapshot images so you will have to opkg install it if you need it. You can install luci with:
opkg update
opkg install luci
Useful guides: