New to Networking; wondering if I could get some pointers

I just started working with a new company doing networking on the side. I work in IT for a company with 6 locations, managed switches etc but I dont do most of that, I’m just familiar with it. This is my first time configuring a network on my own.

This network was setup by Spectrum initially. There is a patch panel and switch, then connected to the switch is a Cradlepoint AER1650LP4 (no wifi capabilities). The Cradlepoint is connected to a Technicolor TC8715D Router/Modem combo and has a split coax with another Spectrum ET2251 Modem.

The company has about 10-12 computers, and it varies if theyre hardwired or on wi-fi. They also have about 10 Ethernet Phones. Currently, the two routers are on seperate subnets and cant communicate. I was wondering if anyone could point me in the right direction.

Do they need both Modems? Would it be better to replace the others with only 1 router/modem combo and run a seperate router upstairs in bridge mode or a Wi-Fi repeater?

I realize the Wi-Fi and Ethernet were separated for security purposes. Can this be achieved in other ways?

Thank you for reading.