If the piece of equipment you got from your ISP is a modem/router combo, but you have another router behind it you are doing double-NAT, which is a no-no. You only want one device doing NAT/PAT, so if there were security concerns with that unit it should be factory reset and setup from scratch with a strong password that it sounds best that the individual we are discussing doesn't know.
The "other" router/gateway can then be setup as an access point and placed upstairs to provide WiFi up there.
To do this:
- Login to the device
- Setup LAN subnet to be the same as the one on the main router/gateway, so if it is 192.168.2.0/24, then you'd set it to be in the same subnet and make its LAN IP something easy to remember for you in case you need to change the config, so, using the aforementioned subnet, I'd make it 192.168.2.254.
- Disable the DHCP server
- Setup and secure the WiFi (WPA2-AES, strong password)
- Plug the cable coming from the downstairs router/gateway into one of the LAN ports on this one
Voila! You have a cheap access point wth integrated switch.
If both devices have WiFi, if you set them up with the same SSID and passphrase clients can less than gracefully roam between them.