Been asked before I know but... need help.
I'm not great at this stuff. I have a Beryl as main router and a secondary flint 6000. My home automation runs on a Raspberry pi and i need remote access to it and various other items in the home including a camera NVR.
Currently I have Tailscale installed on the Beryl with remote access enabled. Tailscale also on the PI (advertising routes 10.0.0.0/24. Is this a conflict? I would ideally like a secondary method to access my home stuff if one were to fail.
thanks
I would suggest changing one of these routes to 10.0.1.0/24 since the raspberry pi is part of the downstream network, meaning the route 10.0.0.0/24 already exists, likely firewalled but the gateway could be accessible due to openwrts ui hosting on 0.0.0.0 meaning it will be visible on every gateway interface like that.
I guess you could theoretically setup another router on your network with a vpn server or tailscale and from there allow access to the raspberry.
Though there will be a single point of failure when the Beryl internet gets down though then those vpn servers are not accessible, you could try to get a second internet via thetering, or a sim modem or via wifi repeater if theres a free network available, but the question is if you want to go that far.
In my situation i choosed to have a backup wireguard server (i configurated it being open to wan to use wan as lan via a open firewall), however only in rare situations it actually helped because many times if i started to configurate to boldly for optimizing it for streaming i managed to cause effects like putting down the full wired stack 
but since you want more access to the raspberry pi, would a ip kvm also be a idea?, many ip kvms have their own lan connection, and the only thing connected to the raspberry pi is hdmi and usb for keyboard and mouse simulation, so theoretical even if the raspberry pi had a bad configuration like not having internet, aslong hdmi and usb survives you can kvm in and you can make that accessible through vpn server.
The routers have the option to multiwan.
My setup is fibre router to a flint and 4 routers connect to the flint in a star network. The flint repeater option is pointed to another router on its own that manages a mobile phone connection for fail over. If the fibre goes off it switches to the mobile network until the fibre comes back on and switches back.
It's almost seamless but not quite, but that is down to fine tuning how sensitive the router is to disconnection without choking the cpu and lines with ping requests..
The mobile also has access to sites that are normally blocked in my country so I have the flint set to scan both lines for dns before selecting and the one that replies gets the connection, therefore bypassing landline restrictions on banned websites..
Thank you both for responses which are very useful. I'm not so concerned about internet failure. In the past year, several times tailscale access has failed for no reason and I had to delete the "Beryl" instance from the Tailscale site and then re-instate it all again. This is not possible remotely. I don't know if the problem is with Beryl router and wondered about installing tailscale on my other GLinet router as well as the Raspberry. I'm a bit lost with the advertising routes stuff though.
Now it has stopped working.
As a retiree I had the pleasure of spending the last 8 hours googling with no success how to get my simple setup working.
Beryl router (10.0.0.1) with tailscale advertising subnets.
I have various devices on that subnet including a pc, 2x Rapberry pi's, an NVR and others.
No matter what I try I cannot access or even ping any device. I have been testing using my phone with wifi switched off. I can connect to the router itself.