Internet Connection Sharing (
ICS) is a Windows service that enables one Internet-connected computer to share its Internet connection with other computers on a local area network (LAN). The computer that shares its Internet connection serves as a
gateway device, meaning that all traffic between other computers and the Internet go through this computer. ICS provides Dynamic Host Configuration Protocol (DHCP) and network address translation (NAT) services for the LAN computers.
Cf.
https://en.wikipedia.org/wiki/Internet_Connection_Sharing
So, that DHCP service may be what is using port 67, and stopping any other Bootp server from binding to port 67.
Is it possible to disable ICS briefly to allow a separate Bootp server to assign the IP address to the PLC, then stop that Bootp server and restart ICS?