I have Spectrum Mobilel with iWatch 5?

iPhone Cellular Carriers that support the Apple Watch, which models, the radio frequencies each model uses, and the country where they offer service:

https://www.apple.com/watch/cellular

They are not one of the listed carriers that support the Apple Watch.

The carrier must be able to share your iPhone's phone number with the Apple Watch. As Spectrum is an NVMO (carrier that rents cell time from another carrier) it is possible Spectrum does not have a contract with Verizon that lets them support the Apple Watch, or their operations setup does not allow number sharing.

I just re-read your message.

The GPS only Apple Watch can do  Everything  the cellular model can do  as long as  your iPhone is  nearby, where nearby is somewhere in the same 3-4 bedroom home (assuming typical wood frame construction).

The GPS only Apple Watch does not care what carrier you use, as it just asks the iPhone to provide the networking services. You carrier doesn't even now that you are using a GPS only Apple Watch.

If you think Spectrum is going to get Apple Watch cellular support, you can get a cellular model, but until you can activate the cellular radio, it will behave as if it were a GPS only.

But since the cellular models are more expensive, you want to be sure Spectrum is going to support cellular during the life of your Apple Watch, or you will have wasted $100 for a radio you did not use.

NOTE: I have a GPS only Series 4, and since my iPhone is basically glued to my hip most of the time, my Apple Watch does everything a cellular model would do.