Easy way is to check what the Bell modem is up to is to see what WAN IP the D-Link has. If it's something like 192.168.1.64, then the modem is making the PPPoE connection and acting as a router, and sticking the D-Link on top of that is technically doing double-NAT which makes port forwarding extremely difficult.

If so, you'll need to change to making your PPPoE connection using the D-Link - this will require the client's login name and password.

As mentioned above you'll typically have to switch the modem to bridge mode for this to work, but not all modems actually require that. I'm using a SpeedTouch 516 on Bell (technically TSI third-party) and it allows PPPoE sessions from my router to occur without switching it to a dedicated bridge mode (which is handy as I can't access line statistics from the modem if the router makes the PPPoE connection, but can if the modem does - so I can switch back and forth quickly when needed).