Dear Johan,
TA offers to download a JSON-file since one of their latest Firmware-Upgrades
https://www.ta.co.at/downloads/datei/17511763-cmi-json-api/
This document describes howto use this service. You can test it with your browser. As soon as you see the results, you can configure the openHAB http-binding and create items with the parameters you desire.
Nevertheless, the parameters you can retrieve are limited.
The 2nd solution (Modbus Master-toMaster proxy) is custom-made by myself and not documented.
It is a NODEJS-server where you can connect to with multiple masters and exchange the data. In this case 1 master is the openHAB modbus-binding and the CMI. Both read and write to my custom-made Master-toMaster proxy.
This solution runs in production since about 6month now without any problem. It exchanges informations from openHAB Homematic, Wifi-Switches and others, the CMI and visa-versa.
On what machine are you running openHAB?