The routers or computers you are using for this have to support forwarding traffic. With Linux this is pretty straight forward for other OSes I’m not sure how easy it is.

You can get around this by having tailscale installed on the default gateway (router) of each network. It might be quite a pain for OP to change routers at each location. On the plus side, OpenWRT has some other cool features like PXE booting.

Here is an article about tailscale on an OpenWRT router.

I’d love to but all the logistics are overwhelming for me. What made your sabbatical so good? Was it easy to finance?