Since I run services that connect in from the outside world, I can’t afford to simply restart my router every time something like this happens. Fortunately, one is able to stop and then start the app independently of other router functions. It’s not ideal and is definitely a workaround versus a fix, but I suppose that’s all I can do before figuring out what’s causing the issue.
It might be worth trying Synology’s bespoke VPN service in place of OpenVPN. It’s reported to be slower, but that’s not an issue given that I only VPN into my home network for temporal admin activities.