Hi
Is it possible to run any routing protocol with your ISP ? Since you are connected both the links onto the ethernet port you cant use 2 static routes to have auto fallback.
In addition to routing protocol you can have a floating static route pointing via tunnel interface with admin distance more than that of the routing protocol configured.
This will help in removing the default route announced by your ISP and install the floating route so that your traffic can flow via the tunnel interface.
regs