Working with another user, and with other model DIR series routers, it seems that DLink maybe using this 192.168.7.1 has there default static Guest Zone IP address thus making this address unavailable for any custom user use. It's recommended that if a user needs this IP address string, try to avoid using it.
There maybe a user modification that could change this by doing a FW modification. This modification is not supported by D-Link and could void the warranty or even brick the router. Avoid this if possible. Ask questions first.
Good Luck.