Disable Guest Network when dual wan in fail over mode?

[butter_fry]

I have a client that needs always-on internet for a small business (for PoS transactions). I suggested a 4g dongle for him connected through the router in fail over/back mode. This is fine, but he also provides WiFi for guests through the guest network feature. When in fail over mode we don't want all the customer traffic running through the 4g dongle eating through what is supposed to be a limited use device.

My question is this: Do you know of a way to disable Guest Network when the Dual Internet is in fail over mode? Might be a simple script, but I just don't know how that would be done.

thanks for the consideration!