You can make this configuration. For this, you need to add the WIFI you want to relyad via Network→Wireless. This will create an interface that you can see in Network→Interfaces. Now to configure the switch between both interfaces you are going to configure a Failover. This way your priority interface will be the new one added via WIFI and the second one will be the mobile interface. Here is a link about the failover configuration.
In the Network→Interfaces menu, you will see which ones are active and their priorities. That is to say, both can be active, but the gateway to the Internet will be given by the one with the highest priority (the one that is placed first in the order).