Hello Barry,
I had a similar experience with the Rangemax stuff where it gave me an almost imperceptible improvement in range. Like you I think the advertising is misleading, as far as I can tell the rangemax products perform *slightly* better at the fringes of their operating range and due to the diversity antenna used in the AP they might perform better around a house which previously had dead spots due to think walls or whatever. But as to actually having a larger range I was unable to confirm.
What I would suggest is returning the rangemax kit as not as described and do the following.
Get a homeplug kit (the netgear stuff is pretty good and I have used the solwise stuff as well) Either get one with 1 Mains to Ethernet converter and one Mains to wireless converter and use the wireless converter to extend the range of your existing wireless in the dead spot.
Alternatively get two mains to Ethernet converters and plug your desktop into one of them and the other into your existing router.
Solwise also make an ADSL router that has a homeplug mains interface as well as the standard 4 ethernet ports. So then in conjunction with one of their wireless plug in units you have wireless broadband where the wireless bit can be easily situated at a different location to the phone line.
We have used this stuff to great effect when running cables would have been an inconvenience and/or expensive and it has proved to give a very reliable connection.
PS. Don't use them on filtered mains strips, plug directly into a wall socket if possible.