Three Firewalla

So I'm currently using the Firewalla Gold Plus for my home network. Everything is super kosher and is super awesome. I've obviously ran into certain hurdles here and there which is normal.

So I'm going to increase my network expansion and buy two purple SE's. 
One is for my mobile hotspot so on the go, I have a VPN connection to home.

The second is the reason for the post.
My mom currently rents from a house owner and her internet is handled through them. However the weird part is both the modem and router are currently installed in my mother's house. I want to use this second firewalla have her connection secured by me and monitored so there isn't issues. The reason I'm asking here is I know this usually not a good idea, but the landlord doesn't want to do things better. Is it possible for the purple to act as a router behind the Spectrum router? Yes, again I'm aware this is not the best way but as I said. The landlord owns and runs this. It's part of her rental agreement and wont budge on doing it differently.

I know it is possible, but I don't know how the firewalla would act and that's what I'm wondering.

Please any feedback is welcome.