LAN traffic not WAN. So yes your internet is limiting but ONLY to the internet. Otherwise a workstation to a NAS/server could be up to 10x faster

You can tell by the IP address range if they're on the same network, normally. If device behind router B can successfully ping device on router A then you're on the same

Start from scratch and do it right the first time. Feed enters home in Basement/Utilities/Comms area. In that area is the main switch where all runs terminate to with at bare minimum 2 jacks at each location but 3 is preferred. 2 allow a router to be placed anywhere, 1 for WAN and 1 for Lan back to switch then a 3rd for an additional device at that location direct to main switch. Extra runs are easy if you're already there pulling wires.

Plan runs for at least 2 locations on opposite sides of the rooms for best layout options. So cables don't have to span doorways windows etc.

Then when cabling is in place a managed switch and router VLANS can be used to separate. Or a 2nd physical switch for the tenant and a VLANS capable router with a port separated to the tenant switch. Then in the tenant space they have the option to plug a router in or you can provide an AP for Wi-Fi. But that will all give you maximum stability, capacity and security.

A vlan router can be as simple as a Thin Client, miniPC or open router appliance with multi port NIC added as needed running OpnSense OpenWRT etc. Those units can be used for anywhere between $25-150 normally and software is free NIC's are cheap

Router 2 should be a switch unless you need the extra stuff for something. At a minimum it should be in AP mode. Otherwise it's very unlikely you're getting "collisions", it's just humming along like networks do

All mine are older but Asus have traditionally been decent. I personally have switched over to using a custom router for none options and capability and a WAP (Wi-Fi Access Point) like Unifi. The WAP tends to be cheaper and generally all one needs to upgrade.

If you're looking for a router specifically a mini pc or open router box can make it easy. Like Protectli Quotom etc. Then run OpnSense on it and will be the last router software you need to learn. Consumer grade stuff is generally mediocre and prices have gotten a bit ridiculous.

If you think you might want multigig LAN ports in the future look for one with the ports or a Thin Client/miniPC/SFF with expansion port and add a NIC to suit your needs.

Should clarify what exactly you mean by not connecting...to the WiFi. Does it connect when plugging in?

So you want us to suggest a device that will connect to your equipment and solve your needs...but you don't tell us what the equipment is or what connection options it has...that's a real problem. If you have an RJ45 port on it you can plug in any normal router or AP with switch.

That "should" work

Any additional routers should be in AP mode or settings modified to run similar. A router performs specific routing functions so there can be 1 setup as router. You can just run a cable to a switch if that's all you need. Or a switch then add a WAP (Wi-Fi Access Pointl to the switch.

This may help clarify a bit for you, there's a lot there so kick back and learn

This will clarify a bit for you...there's a lot there so kick back Home Networking Basics https://www.youtube.com/watch?v=fjRKID2ucPY&list=PLqkmlrpDHy5M8Kx7zDxsSAWetAcHWtWFl

This will give you an overview of what need and how to set it all up. There's a lot of info so kick back

Your best option is use VLANS your consumer router may not do it. So may require setting up an open router and run like OpnSense etc

I believe the highest upload they have is 40Mb, so that's not far off.