GL-X3000 Spitz: VPN only on Cellular

Dear people,

I am in need of your help and expertise. In my home I have a Brume2 as the gateway to the internet running a permanent wireguard connection. I am also running a GL-X3000 as my WiFi. It is using the Brume2 as its WAN gateway. Straight forward and easy setup.

What I want to achieve is that, if for any reason, the Brume2 goes down, the GL-3000X must failover to Cellular. This is also very easy to setup, but I am struggeling with one thing: I want to have the GL-3000X to auto-establish a Wireguard-VPN connection only if it is failing over to Cellular, but not when routing the traffic over the physical WAN port to the Brume2. Alternatively if this is not possible, another solution could be a kind of split tunneling where the tunnel is only used if the traffic goes to the Cellular interface… if that’s possible at all.

So, to sum up:
I want to have either an auto-establishing of the VPN (Wireguard) connection if the GL-3000X fails over to Celluar or to have a permanent Wireguard connection but this tunnel is only used when the traffic will go over Cellular.

I hope I have explained it clearly, otherwise please ask for more details.

Thanks in advance!