DIR-615 releasing IP too often

IP leasing basically says an IP address for ABC PC will be available for that PC when it connects during a time frame. Because your's is set once per month another PC might be trying to gain access to the router for connectivity but be using a static (preset) IP address instead of a Dynamic (randomized) IP address.

Try setting this for 24 hours and run it for about a few days and report back any problems.