COverage area

can a Ap advantage with clusters gave me 20 miles signal range to Subscribers. I need a solution

Depends which frequency. I think only 900mhz can; and for that, check interference first.

We have a 900mhz AP with a hpol omni antenna; our farthest subscriber is 18 miles away with an integrated unit, 40 feet up, good signal.

Can get 15 miles easy if your APs are high enough, and/or you are on the top of the terrain or its flat. With no obstructions, and LOS, I can get 15miles on 5.7 if the conditions are good. Wouldnt think 2.4 could do it because of noise floor. 900 will definitely be able to do it under certain conditions again. You havent mentioned your operating conditions, so the unfortunate answer is, it depends.