Reboot from lan without ssh & luci access

I am not sure if there is any solution other than locally cutting the power in my current situation, but I am asking just in case and maybe for ideas to implement after the next boot.

I have an installation (espressobin) with 19.07 which works fine for 2 years at a remote place. Furthermore, I use overlay so power outages don't harm the system. Two days ago someone went there to change an usb to RS485. After the change, I have no longer access with ssh or luci, ssh does not accept the keys nor the password, and luci says:
Not Found The requested URL /cgi-bin/luci/ was not found on this server.
Nevertheless, both the switch and wireguard work fine.
So I expect that this is some crash that does not trigger the watchdog.
I am wondering if there is any way to reboot the system from a machine in the lan or if there is any other way as wireguard is working.
If not, is there anything that I could implement in order to be able to trigger reboot remotely (from lan) in such a situation (other than periodic reboot with watchcat or usb watchdog)?

You’ll need to physically cut the power if you can’t login via ssh or luci.

What changes did you make? Adding a Usb to rs485 adapter doesn’t seem like something that could cause this issue. (And fwiw, rs485 is an unusual one to use in this context, but the difference vs uart ttl serial or rs232 is just electrical).

Just changed an usb to rs485 with a newer that has TVS Protection (for lightning strikes). An earlier change some months ago of a "fried" usb to rs485 didn't cause any issue.

Is that serial connection going from the router’s usb to some other serial device? Or is the serial connection to the routers uart?

This is from the router’s usb to some other serial device.
Here is the usb to rs485

Now it makes sense why you are using rs485. I’d be surprised if that was responsible for the issue. My guess is that if you had a strike or surge or something that killed one of those devices, it may have also corrupted your routers memory.

Well there was a strike some months ago that killed two unprotected usb to rs485 on two different devices but nothing else (ethernet cables are protected). Since then espressobin rebooted many times, so I wouldn't expect a memory issue, but I will check it once I have access.
Any suggestion for remote reboot or watchdog setup in the future?

Thank you, but this seems too much, as espressobin does not even have hdmi.
A few months ago I was looking to something like this but usually more machines add more complex. On Debian machines, I am using systemd watchdog.