Helium Mobile runs primarily over the T-Mobile network. Coverage is supplemented by Wi-Fi hotspots that are part of the Helium project. These hotspots offer data service, but they don't support normal calls or SMS messages.
The Helium project used to involve a distributed network of CBRS cellular radios. Integrating a small radio network with the nationwide network of a major cellular network involves significant technological and logistical challenges. It appears Helium was never able to pull this off.
When the Helium project had CBRS radios, an awful lot of promotional effort went into hyping up "Helium 5G"βdespite the project using 4G LTE radios.