Your Spectrum home internet connection is IPv4. If you are using the gateway (combined modem+router) that Spectrum provides at no extra cost, it's all done internally, and your public IP will be IPv4.
A customer-provided separate router should be set to accept its network setup parameters from the modem. The data sent by the modem will be IPv4 and again your public IP will be IPv4.
Be sure you remove any IPv6 settings you might have loaded into your computer and/or router while you were experimenting. The router must include ONLY IPv4 addresses for DNS and DHCP settings. Clear any entries labeled IPv6.