I'm using Radioactif.com ISP (that's, I'm from Quebec!)
I've been trying to access the modem's management page, but it is
locked and it's NOT the "Administrator/Blank Password" as the guide
suggests. I have no idea if this works on other models or other ISP,
but here it is... the very simple:

Username: admin
Password: admin

Yes, simple as that and it worked for me. Instantly, after having made
many web searches, I thought I had to put that in the web for others
to benefit. If that might be useful for at least one person, then
great ;) After all we PAID for that modem, it shouldn't be locked!
BTW. The firewall is disabled by default, so... if you're looking for
port forwarding control, somehow, I'm no expert, but the only thing we
can manage is some "Add game/app" which gives us:

"A game or application is made of one or more TCP/UDP port ranges.
Each incoming port range can be translated into a different internal
(local network) port range. Port ranges can be statically assigned to
devices or dynamically assigned using an outgoing trigger."

And then we can enter the following: Protocol | Port Range | Translate
to (wtf!) | Trigger Protocol | Trigger Port

Alright, I hope this could be useful to some!