Just so I'm clear you'll have a router which will be your WLAN which you connect to the internet with.
You'll have another router which is not your WLAN and this does not connect to the internet but your tenants connect to as well as your self.
Most routers have settings for restricting and or allowing.
I'm certainly no networking expert and I'm sure this can get very technical using a bridge connection or something.
Myself I just wouldn't connect my WLAN to the other WLan.
But, how do I view the camera from my independent WLAN. Since it's more and more difficult for me to move from room to room, I have PCs in each room, all of which access my WLAN and NAS-HDD, so I can work on projects, browse the web, etc. from wherever I am. I'd like to have instant access to the camera from all of my PCs including one which connects to my TV so I can browse the web, stream Netflix, Amazon Prime, network TV, etc. from my recliner.
Lacking that, my primary work/web area is a desk with a 23" monitor, It has 4 video inputs (VGA, DVI, HDMI, Display Port) which I can switch between with the push of a button (and can even do picture-in-picture with certain combos). Currently, I have a Mac Mini, W7-32 PC (with swappable HDDs for 10 versions of Linux) & W10 PC connected to the monitor although I seldom have more than two under power at once. The only free video input is Display Port.
If necessary, I can use an RPi to connect to the camera/router but I need to use the monitor's HDMI with my W10 PC as it notifies me that I've got mail via the HDMI audio link to the speakers in the monitor. (BTW, I still use the recorded female voice from my first email account with AOL dating back to when the Internet was in its infancy.) Are there HDMI to
Display Port adapters? Or, are the Display Port to HDMI adapters bi-directional? Even if I find a way to connect it to my monitor, it doesn't help when I'm in another room.
As a last resort, I could add a touchscreen to the RPi so that it's a dedicated and relatively inexpensive viewer for the camera, locating it at my primary work/web area, but, again, that doesn't help when I'm in another room.
One other possibility is to tie the cameras to the WLAN they currently have (or plan) for WiFi security cameras monitoring the garage area at the rear of the buildings (which abut a wooded area where I frequently spot deer grazing). But, this seems to present the same problem of isolating my WLAN (plus additional issues related to the security of their WLAN).