My setup: I have an ER605 Omada router and use an on-site hardware controller. I have three access points connected to this setup and fiber optic internet. This is all located in the main building central on property. My setup is pretty solid. 

The problem: I have a smaller building on my property that I rent out. It has its own ISP connection. The renter pays for their own internet. My access points are strong enough to reach the building, but there's a weak signal inside. You can stay connected to my SSID and use my internet in the building just with much slower speeds. Right now if I want faster speed, I have to manually disconnect my device (my phone) and connect to their SSID. It won't happen automatically because I still get signal from my main building.

My question: I know there are lots of solutions and options for this situation. But I mostly have just one. Would I be able to plug in an access point such as an EAP670 into their router and use their internet and ISP connection. Then configure it to broadcast my main property SSID with my controller controlling it. Ideally I could move around my property and even go into my rental property without ever switching SSID or leaving my network technically while also maintaining higher speed internet access. Is this even doable?

Thanks in advance. Also, if you have other ideas or solutions I'm always open. Maybe there's something I haven't thought of.