Had in interesting issue which there did not seem to be much documentation and needed a bit of troubleshooting. So I’d like to share this with others.
So on a first install of pfSense I managed to get the CARP working on a 4G Teltonika but could not seem to get the CARP working when connected to a DrayTek 2860.
Doing a packet capture on the pfsense showed some issues with ARP. Turns out there is a default setting on these specific models which is only changeable from a CLI prompt on the DrayTek. Later models have it in the GUI.
More info here so suggest a read and understand if you want to allow this or not.