I found that my mobile data provider uses IPv6 for their APN. Assumption: the R1 does not (fully) support IPv6.
KPN offers an alternate APN that uses IPv4 addresses instead of IPv6.
To fix the hotspot connection issue for NL data providers that use the KPN network do the following:
- On your iPhone or iPad, go to Settings > Cellular > Cellular Data Network.
- Tap Cellular Data Network.
- For the Personal Hotspot section, change the APN from “internet” to “portalmmm.nl”
- Tap back
It should now connect normally.
Inspiration: kpn support page
For t-mobile you could try pcweb.tmobile.com as the APN setting.