Not sure, what you are exactly searching for. I’m one developer of the Eclipse Open Source projects Californium (CoAP/DTLS 1.2 CID server in java) and tinydtls (DTLS 1.2, device C-library, client-side CID). In order to have a “e2e” integrated setup, I started a couple of demo projects:
Zephyr - coaps-client using tinydtls
This device client is in the meantime pretty reliable. In rare cases the watchdog will reset it, but usually it runs for a couple of weeks without such restarts. The power and data consumption is pretty low, a Thingy:91 is able to exchange a message (e.g. 500-800 bytes) every hour for more the 6 months. (OK, that depends a lot on the radio situation where you want to run it. The short: it takes about 60mC to exchange a message, but easily 2000mC to search for a network.)
A nRF9160 feather runs from 3xAA 2000mAh LSD batteries up to 2 years.
See Virtual IoT & Edge Days - Day 2., DTLS-CID Meets Zephyr, at 2:45
CoAP and DTLS 1.2, DTLS 1.2 CID are all IETF standards, so it’s possible to exchange components supporting the same standards. e.g. you may enable DTLS 1.2 CID support on the nRF9160 modem, or use Golioth as CoAP/DTLS 1.2 CID server alternative to Californium. IoTerop offers also a CoAP/DTLS 1.2 CID server endpoint.
What’s not standardized on the CoAP level is the API the server offers. If you’re looking also for standards there LwM2M may be a choice. There you may find a couple of server, e.g. IoTerop, AVSystem, or Friendly Technologies. Or as open source, Eclipse/Leshan.