ZeroTier - Using an OpenWRT router to access a home ISP for internet traffic

Ok, so what I am trying to achieve is this:

I want a wired & wireless network in a remote location which uses the ISP in my home location for all traffic.

I have an internet connection in the UK. The ISP's router is still in use, but I have an OpenWRT router (bthhv5) connected via LAN, and with Zerotier installed.
I've given this 192.168.191.1 in the zerotier config.
I have zerotier installed on my laptop, and if I tether this to my phone and then:

route add 93.184.216.34 mask 255.255.255.255 192.168.191.1

and then tracert 93.184.216.34, it takes me out via zerotier, in through my OpenWRT router, routes the data through to my ISP router, and hits example.com.
So all seems good with the 'receiving' router. I can use a zerotier client, and route back through my home network, and out to the internet.

So, now I take a second router (again, bvthh5).
This router will be in a remote location. Again, the ISP's router will be in place, but this time I intend to use the ISP's router's lan as the WAN port on the OpenWRT router, and the LAN and wireless should be routed via zerotier.
(I may have two LAN ports which route to local... but I've had that working :slight_smile: ).

I want to have the brlan route all traffic via zerotier.

At this point I'm struggling with the configuration required.
I need brlan to route all traffic for 0.0.0.0/0 through zerotier (to 192.168.191.1). But I don't really understand how to do this without zerotier trying to route itself through itself?
And it seems oh so easy to lose the default route?

Has anyone got a working version of this, and able to share their configs or provide advice?

best regards,

Simon