I found a weird solution to this. I was getting those IP changes frequently. When my ISP got me to connect the router to my test port I was sceptical, but the problem went away! Eventually I realised that a non-functioning phone extension was wired into the master socket. It wasn't filtered as the filter had been at the other end. It must have been acting as an aerial picking up interference, raising line noise and provoking the ISP's adaptive noise management to redo the connection.
Probably not your issue, but maybe worth thinking about.
<Edit> More info...
I still didn't really believe it, but being an amateur scientist I tested three times, for two days each time. Test port - problem went away; normal port - problem returned. (I didn't tell the ISP I was doing this!) Eventually I removed the old phone extension and the problem went away for good; IP changes are now less than one a month.
BTW most ISPs don't change your IP daily unless they have to as in my case.
I understand the reason some games don't allow IP changes is that accepting them would open the door to exploits involving handing over the game connection to "special" software. I guess that this is more threatening for a game based on P2P architecture, although I'm not an expert here.
