[Lord Hasenpfeffer]

There is a difference in my router config compared to what's shown in the page at the URL you cited. I have fields for "External" starting/ending ports and "Internal" starting/ending ports. These options and my settings are as follows:

Service Name: Transmission
Service Type: TCP/UDP
External Starting Port (1~65535): 51413
External Ending Port (1~65535): 51413
[*] Use the same port range for Internal port (CHECKED)
Internal Starting Port (1~65535): 51413 (Note: this is preset because the above option is checked.)
Internal Ending Port: 51413 (Note: this is preset because the "same port range" option is checked.)
Internal IP address: 10.0.0.2 (Note: this, my PC, is the only currently connected device)

In the UPnP section of my router config, I have:

Turn UPnP: "On"
Advertisement Period (in minutes): 30
Advertisement Time to Live (in hops): 4
UPnP Portmap Table:
Active: YES | Protocol: TCP | Int. Port: 51413 | Ext. Port: 51413 | IP Address: 10.0.0.2

In the Preferences->Network tab of my Transmission client I have:

INCOMING PEERS
Port for incoming connections: 51413
[Test Port] (Note: This is a button. When I click it, I see "Testing..." and then "Port is closed."
[ ] Pick a random port every time Transmission is started (NOT CHECKED)
[*] Use UPnP or NAT-PMP port forwarding from my router (CHECKED)

In the Preferences->Privacy tab of my Transmission client I have:

BLOCKLIST
[*] Enable blocklist: http://www.example.com/blocklist (CHECKED)
Blocklist contains 225,033 rules [Update]
[*] Enable automatic updates (CHECKED)

PRIVACY
Encryption mode: [Require encryption] (This is a select list with 3 options)
[*] Use PEX to find more peers (CHECKED)
[*] Use DHT to find more peers (CHECKED)
[ ] Use Local Peer Discovery to find more peers (NOT CHECKED)