thanks for the suggestions from you and the others...

As indicated in earlier response, I believe my core problem is a wiring
error; repair and testing must wait until I can get parts on Monday.
HOWEVER, I am at a loss to explain why the error is a problem for the
D-Link DI-524, yet it had not precluded operation of several different
lap top computers.

On the use of straight through versus cross over cables:

I wrote:
> >One of the LAN outputs of the first router is connected to one of the
> >LAN outputs of the second router so that it acts like a switch.

You responded:
Can I presume that you used a cross-over Ethernet cable between the
LAN ports on the two DI-524 routers? ...LAN to LAN port connections
must have a cross-over cable unless ONE of your boxes has
auto-configuring ports. ....The DI-524 does NOT have port
auto-configuration and the cables must be cross over.

The tests that I did at locations #1 and #2 used straight through wired
cables. I repeated test at location #1 this morning and indeed confirmed
LAN to LAN works just fine with straight through cables. Subsequently I
found that my DI-524 manual (pg 5) states "all Ethernet Ports are auto
MDI/MDIX .."

I also enjoyed reading of your various escapades related to installing
Ethernet links in your 05/03 9:29 AM post. I can identify with them as
years ago I worked on the staff of a commercial radio station where the
transmitter was on the top of a hill in the middle of a frequently
snow-covered cow pasture. I can remember using lots of unorthodox
methods to get the signal back up and on the air. (I do have historic
First Class FCC license which is of little use today.)