It isn't easy to make any suggestions without knowing more.
I assume:
- You are using the Meraki GO gateway (GX-20 or GX-50) with a static WAN-IP on your Spectrum modem. If not it might be worth to give that type of connection a try.
- Phones are connected directly to the Spectrum modem and are not part of the Meraki GO network.
I assume that, because I have a similar setup at home with my cable ISP.
I don't see how a secondary internet connection (Starlink or Nighthawk) will fit in that type of setup, since the Meraki GO gateway offers only one WAN-port. You would have to switch the connection manually and you might have to change settings on the gateway every time you do that. Not knowing which connection is stable or not at the given time.
When you say they are working on an issue locally, this issue happens at your place or in your area? Did you ask Spectrum to replace the modem?