Wifi Antenna problem

First of all as I am brand new to this forum greetings everyone,

I have a small problem that I am trying to sort for a friend he has a chalet block of 18 rooms starting 20 mtrs from his main building up to about 70 mtrs away the internet line and moden router are on the top floor office of the main building which is approx 40 ft above ground level. He was looking to extend his wifi range to the chalets and surrounding grounds by detaching the 2 dbi antenna that came with the netgear g modem router and attaching an external 15 dbi omni antenna to the outside of his building using 15mtrs of low loss cable.

He purchased all the required items Antenna, pigtail, low loss cable and set it all up on the roof but is getting next to no signal from the new antenna 1 bar on laptops and phones up to 100 meters away outside but no signal beyond that or in the chalets and unless you are standing right next to the antenna you cant get an excellent signal.

He is using

Netgear modem router G DG834G 54mbps which comes with a detachable 2 dbi antenna.

TP Link tl-ant 2415d 15dbi external omni Antenna which is 4ft long

I did some checking and the reviews I found were all good for the antenna for improving wifi coverage

Is it possible that the router is not putting out enough signal strength to operate the antenna properly or is it something else am at a loss on how to fix this as I don't want him throwing money at it in the hope of a fix.

Many thanks in advance