As understood, you are currently connecting to the Garmin GLO via WiFi; the iPhone is obtaining its internet connection via Cellular - and you are tethering the iPad to the iPhone with a wired connection. As is, simultaneously powering both iPad and iPhone from an external power source using this arrangement may be challenging.
Instead of attempting to tether your iPad to your iPhone's hotspot via its USB-C port, you might instead try pairing the iPad and iPhone via Bluetooth - yours being one of the "edge cases" where this connection method may be advantageous...
To do so, you first need to manually enable the hotspot feature on your iPhone:
Settings > Mobile Data > Personal Hotspot > Allow Others to Join - set to ON
Next, Bluetooth for both the iPad and iPhone must be placed in Discovery mode:
Settings > Bluetooth (accessing this page will automatically place the device in Discovery mode)
Whilst both iPad and iPhone are in Discovery mode, you can mutually pair the devices.
Once paired via Bluetooth, your iPad's USB-C will be available for powering the iPad from a suitable USB power source - leaving WiFi available, as now, for the wireless connection to your Garmin GLO. You will also need to power the iPhone from a USB power source via its Lightning port.
