From what I understand, you should not set up the upstairs router as a router, but just as a wifi access-point
In that way it will just be one network, re-using the dhcp from the main router 
If your router is not capable of acting as AP, maybe re-flash it with OpenWRT or DD-WRT?
I use both my old isp routers as wifi AP’s using both openwrt (on a D-Link) and ddwrt (on a linksys), as my EdgeX router doesn’t have wifi capabilities 