Looking on the Linksys site, the device should have a dedicated WAN port, which leads to the modem, if the modem isn’t connected to the Internet and the Linksys isn’t connected via the WAN port to the modem, it won’t get Internet.
If you aren’t getting assigned an IP address over wireless, then I would guess the DHCP isn’t configured correctly.
Can you still connect via cable? Do you get an IP address then?
If not, you will need to look at the documentation and see what the default IP address range is and manually give the PC an address in that range, so that it can connect and you can work on the set-up further.
Also, Windows (and most other OSes these days) often fail to connect to an access point, if it doesn’t provide internet. You need to force it to connect in those circumstances.
But I’d go back to wired first and ensure that the Wi-Fi is set-up properly and that the DHCP settings are correctly apportioned to the main and guest networks.