I just checked a few MGs...
Two MG51 devices both report connectionType "lte" in the API response, in the dashboard view they both show "4G" and "5G" on the "uplink" page.
Also checked an MG41E, it also has connectionType "lte" in the API response.
In the case of the MG51 devices, both report a 'great' 5G signal vs. a 'good' 4G one, so I'd assume they are using 5G radio.
The MG51s and the MG41E all have signalType None.
Also have an MG21E currently offline, it's last reported status also has connectionType "lte" in the API response, but it actually had a signalType "4G".
Based on that, it looks like the API maybe isn't correctly returning signalType in all cases.