Found the problem! Hurrah!
Managed to find this page on other DHCP Settings for routers which listed instructions for Cisco: https://wiki.fogproject.org/wiki/index.php?title=Other_DHCP_Configurations
We initially used:
Option 66 [Server IP]
Option 67 ascii undionly.kpxe
What actually worked
next-server <Server IP>
bootfile undionly.kpxe
Thanks for the help @Sebastian-Roth @george1421