@IzBox 

If it works, it works.

Thank you for the solution.

@IzBox 

Replying to this old thread for other users' future reference.

This issue is not caused by the modem management interface at 192.168.100.1.  Instead, it's caused by a hidden "auto-detection" feature of the TP Link router which interprets the presence of DNS and DHCP service running on another host within the same subnet as an indication of a duplicate gateway/subnet conflict.  It's intended to avoid causing existing networks from being taken down by a non-technical user plugging the router into a network with an existing gateway.  The TP Link router will automatically change it's LAN configuration to a different subnet to avoid the collision.  A cryptic error message will be displayed, "To avoid IP conflict with the front-end device, your routers IP address has been changed to X.X.X.X" but the real explanation won't be given.  As you've explained, you are deliberately running a separate DHCP service (and presumably DNS too) within your network. This should be permissible but the router won't tolerate it.

I've posted about the issue in more detail in a separate post:  https://community.tp-link.com/us/home/forum/topic/516726?replyId=1031064

That said, your workaround to set the router subnet mask to /16 (255.255.0.0) will not be suitable for any user/network that requires subnetting (e.g. using VLANs).  An alternate approach is to ensure that your TP Link router is configured to use a DNS other than the one colocated on your DHCP server.  This had the disadvantage of not funnelling all DNS queries to your private DNS, but it would enable use of proper /24 subnets.