When I try to remove it from the network via the thermostat itself I get the same error as the OP… The IQ2+ and the thermostat are wall mounted so moving the panel closer to the thermostat is not practical. Please help me figure out how to get the panel back on the ZWave network.
While the thermostat may no longer be showing on the panel list or in Alarm.com, Z-wave devices save their parent network connection on board. That must be cleared using the exclusion steps on the panel before it can be re-added.
For the IQ Panel 2, follow the steps in the link below, including the Remove Device steps first. Those steps clear the internal data on the thermostat.
Bringing the thermostat closer may or may not be necessary, if you perform the steps in the link above with the devices in their current position is it successful?
Both the IQ Panel and the thermostat can be run on battery temporarily, you could bring the thermostat closer to the panel as a test to make sure it successfully clears during the process above, then relearn it, if you do not want to bring the panel closer. It would be easier to bring the the thermostat to the panel.