Twin SIMs are not a common practice in the US, since providers want to bleed you dry. That said, you are describing what typically happens when the APN is not properly configured with a single SIM.
In many cases, the APN is already present for “known” providers, but some will have to be loaded to the device. With a duplicate SIM and an uncommon device, that is usually done manually.
Assuming all of these watches use the same base, it would be located in:
Settings -> Connect -> Cellular networks -> Access Point Names
If your provider has the settings on their webpage or support forum, you may want to compare them to the ones on your watch. It may save you a long wait on hold or a trip.