Hi Kerry , I didn't have a router when I was using the Siemens . I think that bridge mode does sometimes slow the speed . I will see if I can find something on that.
The only things I can suggest unless there are some router settings for this router
are try a different ethernet cable between the router & modem or a different jack than # 1 on the router. Also try the router with just the one computer connected .
If you aren't already doing this set your computer up with an internal static address . I haven't used briged mode so I'm not sure about this but you may even be able to set the router with an external static IP address . Then let the modem work with the Dynamic IP address that AT&T uses unless you pay extra for a static IP address.
I also use one of the Google DNS servers & one AT&T DNS server. So far this is working fine.
I don't know how you tested the Download speed but the best way is to download a large file to your computer . The speed test sites like to make results look good so you keep coming back .
Also : How many lights does your Siemens 4100 have ?
There should be some additional model numbers too . At least I think there are.