Two RBR50 routers, + one satellite

The most obvious solution is to have 2-4 ethernet cables from the basement to the upstairs.

• One cable to connect the modem to the upstairs router, and either
• One cable to connect a switch in the basement to the second floor, or
• 3 cables to connect those devices in the basement to the second floor.
• The material cost is minimal, but installing ethernet cables may be prohibitively expensive.

A lot depends on whether WiFi service to the basement is required (or even 'desirable').

If WiFi is not required in the basement, the the router->AP solution will work.

• Modem is connected to router 1 WAN port in basement
• 2-3 devices are connected to LAN ports in basement (either directly or using an inexpensive switch)
• LAN port on basement router is connected to WAN port on upstairs router (using the single ethernet cable)
• Upstairs router is put into Access Point (AP) mode so that downstairs router assigns IP's to both downstairs devices and upstairs devices and all devices are in the same IP subnet.
• WiFi on downstairs router has power level reduced to minimum possible and 2.4G WiFi on different channel than upstairs AP router. (Note: I believe this totally eliminates any barrier between primary and guest WiFi on the upstairs Orbi, because the basement router is unaware of any distinction in packets that pass through it.)
• Material cost is one inexpensive switch. No installation cost.

One user has overcome the "only one ethernet cable" problem by using two managed switches and VLANs to use the single cable to carry both the WAN and LAN connection.

https://community.netgear.com/t5/Orbi/Disabling-automatic-subnet-reconfig/m-p/2031354#M110164  

• There are two VLANs.
  #1 carries only modem to WAN packets
  #2 carries only LAN to device packets
• On the two managed switches
  Port 1 is connected downstairs to the modem, upstairs to the router WAN port
  and is defined to carry both VLAN#1 and VLAN#2
  Ports 2-8 are designated to carry only VLAN#2
  One of the upstairs ports is connected to a router LAN port.
  The remainder of the upstairs and downstairs ports are connected to user devices
• This solution would allow an Orbi satellite to be installed in the basement to provide totally seamless WiFi to the entire house.
  (Even if the basement Orbi has the identical WiFi SSID/password, the two WiFi systems will remain separate in the sense that devices will not roam seamlessly.)
• Material cost for the Netgear GS108Ev3 switches used by this user is about $200. Installation cost zero.
  There are less expensive managed switches. The TP-Link TL-SG108E is selling for about $30 on Amazon.
  That would reduce the material cost to under $100.