Frequent disconnections from SMC wireless router

Hello,

Our home wireless network is operated by an SMC 2804WBRP-G Barricade
wireless router, which is connected to the cable modem.

Internet access through the ethernet interface works just fine when
tapping both directly from the modem and from the router. However, I've
been experiencing frequent disconnections when connected through a
wireless interface.

I plugged an SMC EZ-connect unidirectionnal antenna into the router to
cover more area (which is more or less effective; the signal suffers
much from the presence of many walls). Also in the router
configuration, I've enabled 'Long range mixed mode signal (11b+11g)'.

If I disable 'long range mixed mode' in favor of just 'mixed mode
(11b+11g)', the disconnections seem to go away.

Is there any reason why this should happen? Why the distinction between
long-range and not-long-range? Is there anything I am doing wrong in
the setup of the router?

Many thanks in advance,
Dominique