A 3rd-party router could potentially work on Spirit network, as long as the router is a stand-alone ethernet WAN router, and NOT an ADSL modem/router or WiFi AP. However, Spirit will not be responsible for configuration, support or performance of BYO devices.
For the setup, ensure these settings are configured:
Connection Type: Ethernet WAN
Doesn't require authentication
IP Address: DHCP (IP is obtained Automatically)
DNS: Either Automatically obtained OR 188.8.131.52 & 184.108.40.206
For 3rd-party device support please refer to the original manufacturer of the equipment.