Bridge mode vs. router mode in DSL modems

[Jason Hsu <jhsu802701@jasonhsu.com>]

QUESTIONS:
1.  How do I know if my DSL modem is the culprit blocking remote access to my computer?
2.  Exactly what is the difference between bridge mode and router mode in a DSL modem?  I read that if my DSL modem is blocking remote access to my computer, switching it to bridge mode would remedy this.
3.  Why does switching my DSL modem to bridge mode cut off Internet access, and why does switching it back to router mode restore Internet access?

BACKGROUND:

I have a small home network.  The setup is:
Internet -> DSL modem -> Firewall/server computer -> Ethernet switch -> Main computer

The DSL modem is an Embarq EQ-660R ADSL router.  My ISP is CenturyLink.

I'm trying to set up an SSH server on the firewall/server computer.  I have a free account from DynDNS, but their Open Port Tool (at https://www.dyndns.com/support/tools/openport.html , which I set to port 22) gives me the "timed out" error message.  I don't think the Shorewall firewall on the firewall/server is the problem, as I have the /etc/shorewall/policy file set to accept firewall-to-all communications (through port 22) and the /etc/shorewall/rules set to accept net-to-firewall (through port 22).

Some searches on Google gave me the idea that my DSL modem could be the culprit.  This brings me to the questions at the beginning of this post.

-- 
Jason Hsu <jhsu802701@jasonhsu.com>