You aren’t limited to only your ISP’s router. Purchase a wifi access point which offers more control over the wifi settings.
Here’s one for $30.
Just plug the new AP in to your existing router’s LAN port.
On the new access point, turn off DHCP so it doesn’t try to hand out IP addresses to your gadgets. Let the Spectrum router keep doing that job. Turn on the 2.4 GHz radio only, and experiment with the channel settings: channels 1, 6, and 11 are the good ones. Make sure 2.4 GHz bandwidth is set to 20 MHz.