I could be wrong but if I had this problem, I’d establish a managed route.
Assuming your LAN router gateway is 192.168.1.1, and your ISP router is 192.168.15.1, I’d put the route into ZeroTier Central as 192.168.15.1 via 192.168.1.1. This might work if you have your subnet bridged into ZeroTier correctly, and I’m not familiar with doing it at the router, but I figured I’d recommend the option.