OpenWrt on Ubiquiti EdgeRouter 4 (ER-4)
Mid-2021 I noticed it seems like Ubiquiti has quietly discontinued their EdgeRouter lines (they're increasingly sold out, there haven't been firmware updates in a while), and since I like to keep my devices' firmware up to date I became a little concerned.
In looking for solutions I eventually found I could extend the life of my EdgeRouter 4, which is otherwise pretty solid hardware, by installing OpenWrt. Luckily I already had a USB-to-serial cable on hand, and with that it was pretty straightforward, following the OpenWrt project's instructions for EdgeRouter devices.
I used KiTTY, with serial speed set to 115200 per the instructions (and per Ubiquiti).
I encountered one hiccup: where the OpenWrt instructions suggest you mount the USB device after first booting in RAM, I found I could not access the USB device to mount it.
The solution: at this point, you basically have a full OpenWrt instance running in RAM, so you can have your WAN port connected to your internet access, connect another computer to a LAN port for a 192.168.1.x IP, do opkg update; opkg install openssh-sftp-server — and then use scp to transfer the sysupgrade file to /tmp/
After that, it's been pretty straightforward using OpenWrt on the EdgeRouter 4, and fun to have more control over this device than I ever had with Ubiquiti's custom OS!