"UPnP is enabled(as well as on router)"
That is not a modem, modems don't have firewalls/UPnP support - what you have there is a gateway, modem/router combo.
Look on your WBR14 for what its WAN(internet) IP is - if its 10.x.x.x, 192.168.x.x or 172.16-31.x.x then its behind a NAT (router) and for port forwarding to something behind the WBR14 you would have to forward said port to the WAN(internet) port IP on your WBR14 on your "modem"
This is a double NAT, ie a public IP changed to a private twice (double) This is not normally a setup that you would want - look to see if your modem can be setup in bridge mode. So that your WBR14 gets a PUBLIC IP on it.
Or you could just use your WBR14 as a accesspoint (wireless to your network) - this is done by turning off the dhcp server on your wbr14, and connecting it to your "modem" via a LAN port on your WBR14 -- give the WBR lan an IP on your "modems" network 192.168.?.? and then you can connect to that IP to setup the wireless.
Other option if you really want to use your WBR to control port forwarding is to put the WBR wan IP into the dmz of your "modem"
Again if it does NAT its not a MODEM.. There is a lot of confusion of terms out there with terms. Here is what they should be called.
Modem - changes media type, say cable to ethernet - pubic IP is given to device connected to it.
Router - in the home market this is a device that takes public IP on wan interface and allows multiple devices to plug into it on the lan side using private IPs and share the 1 public IP with multiple computers on private IPs
Gateway - this is combination of modem and router. Most every single ADSL, DSL device sold today is a gateway device. You only normally see true modems in the cable internet market.
These are how these terms should be used when discussing home internet connections for sure. If you tell me you have a modem, I would assume you have a router behind it or only 1 device connected to it which has a public IP on it. Quite often user is doing double nat because what they call a modem in front of their router is really a gateway
If you tell me you have a router I would assume you have a device in front of it. If you tell me you have a gateway tells me you only have 1 device that converts your internet connect to a private network. Rarely have to worry about double nat.
Hope this helps.. I would suggest either putting your "modem" into bridge mode which means it really is just a modem, then your router would handle any PPPoE type logins, or put it into half bridge mode where it does the PPPoE login but puts a public IP on the device connected to it. Or just use your WBR as an access point. The double nat option is not the preferred choice - you can run into issues with things behind double nat. If all your doing is outbound browsing the internet, reading email it works not too much of an issue - but if your going to want to do port forwarding then yeah double nat can be problematic.
I have see triple nat and even quadrupedal nats out there -- because the user keeps connecting routers to add more ports or more wireless when all they really needed was switch or accesspoint, etc.