In setting up MRTG to work with the NTI Enviromux Mini, I could not for the life of me get it to properly see the vendor's MIB.

So, if you're in the same boat, just change appropriate lines in the mrtg.cfg for your enviromux on the Humidity and Temperature lines to match these, to use the numeric OID instead of the name:

Target[humsens_1]: 1.3.6.1.4.1.3699.1.1.3.1.3.1.0&1.3.6.1.4.1.3699.1.1.3.1.3.1.0:communitystring@ip:::::1

Target[tempsens_2]: 1.3.6.1.4.1.3699.1.1.3.1.2.1.0&1.3.6.1.4.1.3699.1.1.3.1.2.1.0:communitystring@ip:::::1