OK I have looked deeper into this - got a few things on here so trying to focus.
It seems that everything with a DHCP lease is behaving perfectly. What is not working is items that I have a fixed IP for, which is anything that is essential to be stable on my network (fileservers and so on). When I used PFSense, I would create static addresses for the items that had a fixed IP as well, and that always seemed to work. With the Teltonika, it doesnt seem to work. On PFSense I would set the MAC, IP, ClientID & Hostname, with the Teltonika I only have MAC, IP & Description. Could this be the problem where the issue lies?
I suppose I could fix everything to the static address I want using static leases & put them back on DHCP, but I would prefer if the Teltonika was offline that all the essential stuff stay on its known IP addresses.
Appreciating this help, thanks!