Referring back to this thread...Thanks to Gary and msg once again, for their suggestions. I've restored all of my home network, with my Linksys router+WAP behind my new Actiontec DSL modem+router.
I found an 18 VAC power supply in my box of old electronics, but unfortunately it was dead. I also tested the power supply from the old Speedstream modem with my voltmeter -- and got 21.6 V? Is that out of spec? I thought that my voltmeter was true RMS -- it measured115V from the AC outlet.
The key to getting things working: I disabled the routing features on the Actiontec and ran it in transparent bridging mode. No one ever came out and told me that transparent bridging mode was the best way to emulate my old modem, and I read dozens of web links trying to determine whether this was true. But I eventually became comfortable with the routine of switching cables back and forth, and performing the factory reset on the Actiontec. I decided I could experiment at will, and eventually I got all the settings correct.
I was mistaken when I said that the modem offered two transparent bridge modes. There are two modes for managing IP addresses, when two conditions are true: 1) you are operating the modem as a transparent bridge, and 2) you have a static IP on the WAN side. That didn't apply to me.
I think that I'll keep the Actiontec modem, even though I initially considered it too smart for its own good. Unlike any used modem I might buy, it does have a warranty. The router inside the Actiontec box will also serve as a backup hardware firewall, in the event that the Linksys ever fails. Furthermore, I'm running mostly Linux on my terminals, and I don't know how to configure PPPoE on Linux yet. Again, in the event that the Linksys fails, having the ability to put the authentication information in the modem makes some sense.
Now, even though everything is functional, there are three more issues I would like to address.
First, there is the mystery of why the modem is working at all right now, even though the Internet LED remains off. When I connected the modem directly to a single computer, the Internet LED was green.
Second, I noticed a significant reduction in browsing speed once I got the router back in place. Now, I'm still getting my full DSL bandwidth (1.5 Mbps download and 384 Kbps upload), once a lot of packets are streaming to/from a single source. However, from watching my web browser run, I can see that DNS lookups take considerably longer when I connect through the Linksys than when I connect straight to the Actiontec. A web page with lots of links loads very slowly. Where I should look for a performance bottleneck?
Finally, I would like to access the modem from my local net, if that is possible. Swapping cables, rebooting, and reprogramming gets tedious. But the modem is on the WAN side of my router. The modem has a default local IP address of 192.168.0.1 and is accessed through an HTTP interface. With the Linksys router in between my terminal and the modem, the web browser just hangs trying to access that IP address. Any suggestions?
Thanks once again for reading!+-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-+ | Ladasky Home Solar, Inc.: blowing sunshine up your | | power grid since March 24, 2005. Fiat lux! | +-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-+ | Uptime Downtime kWh generated kWh consumed | | 939.4 days 14 hours 17188 17532 | +-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-+