All kinds of DTU which support register packet can be used as access devices, and can be selected depending on different uplinks.
If you don't have a commercial DTU at hand, you can realize DTU function directly with Python code.
Recommend this tcp2serial open source：https://github.com/lewei50/DTU/tree/master/python
Commercial DTU
GPRS DTU
Recommend URS730S.
Ethernet DTU
Recommend URS730S.