The DISABLE_ETHERNET macro is interesting, but not available at runtime. I'm presuming TEST_NET_IF is a secret function at this point.
And importantly, where are all such available macros for the Project Options described?
We did get a response from Digi support:
"I didn't find any references to TEST_NET_IF in the libraries. If they want to turn off Ethernet, they have two choices:
1) Define DISABLE_ETHERNET in the project settings to exclude Ethernet support at compile-time.
2) Use pd_powerdown(IF_ETH0) to turn it off at runtime. The sample LowPower/LP_SMTP.C demonstrates turning power off and then back on (in this case, with Wi-Fi, but the process is the same for Ethernet).”