I have just run my old spigot 1.12.2 server on my gaming pc (high spec so no hardware issues) with the ethernet cable plugged in to the modem.
Download was 333.33 Mbps and Upload 37.30 Mpbs, but still the server is unplayable.
The guy from the isp who connected everything a few years a go did some wierd things, he configured the extra router as a bridge modem and than a long ethernet cable under the garden to the router in the shed. Can't you plug that cable straight into the modum and all the local ips stay the same.
I don't really want to reset every thing, because I don't know if it will work again.
This is my setup, maybe that helps