How to force Win 8 box to use a given WAN gateway?

Question

I have a laptop that I connect to via RDP on a 192.168.. LAN via ethernet. The box that I'm connecting from owns the WAN connection for the LAN. i.e. the router is not configuring the internet connection for the LAN, this box is, and having the router manage the WAN connection is not an option.

So that all works but sometimes I want the laptop to use my phone to access the internet via usb connection and most of the time that works too. However every now and again the laptop switches over to trying to use the LAN router as the gateway for the internet connection instead of the phone.

As above this doesn't work because the internet connection is not on the router but on this machine and no I don't want to use ICS. So it just times out.

When the laptop decides to use the router as the gateway, I can't seem to force it back into using the phone as the internet gateway instead.

So I have to reboot the laptop.

I have set up the phone IP as a gateway option in network connection properties. But in order to be able RDP into this laptop I must also have the LAN router IP address in there too. So i have two gateway entries for this ethernet connection on the laptop. One for the LAN router and one for the phone.

But once the laptop decides to use the LAN router as the gateway IP, I'm stuck and so have to reboot the laptop. I've mashed around with:

ipconfig /renew

but the laptop is stubborn. Restarting the hotspot on the phone does not help either. The laptop recognises the phone but wont use it to get out on the net.

So how do I avoid the reboot and force the laptop to look for the open pathway to the internet via the phone IP?