Thank you for responding @coatmaker618
Yes, I guess there are three questions in there so thanks for splitting the questions up for me.
I think I have gotten a little further towards getting some information on this. I have got my pfsense to recognize a USB device as a WAN port which I can configure. I now have to configure the multi wan part now.
As for the third question, yes that's the interesting one and I am not having any joy finding any help on that one.
The scenario is as you describe. I don't want all my devices to take advantage of the failover WAN because it will be a 4G device with just a tiny amount of data allowance on it. I will keep it topped up for the eventuality of someone cutting up my cable internet while I am not at home so the home can still talk to me and let me know what is going on.
For the failover, out of the 50 or so devices that currently use the internet, I only want to allow my IP Camera NVR, my home security system and smart door lock to have access (all with dedicated local IPs). This is to get notifications of any issues while I am out of the house and also jump onto my IP camera feeds if needed just to see whats going on.
Hope this paints a good picture of what I intend to do.
So if there is anyone out there who knows how to handle multi wan ports in this configuration, please let me know.