Since I have no patience, I’ll lay out some items for consideration.
1st, I wouldn’t rely on an ISP router to serve as my end point for a VPN. They likely have access to manage that device and it likely isn’t getting any updates. You are better off implementing it with your own equipment that you keep updated.
If you have a capable security device serving as your router to the external internet and you want full access to your internal network, then you might consider using a VPN that terminates at your router.
I myself am a fan of setting up a jump host and initiating a VPN connection directly to that host when using an agent based solution. Then you can monitor the host for activities, more easily keep your edge device patched, and then use the capabilities of your jump host to interact with the rest of your network. This would require either an agent to periodically poll a platform for connection requests or another form of ingress into your network.
Good point about network availability and endpoints.