In order to fix bell advanced dmz, after R3 start or restarts, you just need to disable BELL DMZ , save, and then re-enable again , then it works, yes, its weird but there it is. you do not need to remove device or uncheck ADMZ box during this process.
I myself am using PPPOE which doesn't need this superduper advanced "technique" , you will need you bxxxxxxx user name and password which you can reset on bell cs pages if you dont know it , go to mybell, internet serviceb1xxxx profile and change password, you would need to re-enter it in your modem as well afterwards...
I had this issue with routers before R3 too....