I am reasonably new to powerlifting, the first routine I followed was a split routine, 3 days a week, 3 different lifts and related assistance exercises. I followed it for good 14 weeks and that is good time spent for a single routine. I am still experimenting. I achieved my goals through hard work during the split routine.
Now I am following a different routine, 3 days week, all 3 lifts everyday. I am in my 4th week now and the initial cycle is of 8 weeks. I must say, that the new full body training routine is more challenging and more rewarding. This is my personal opinion. I do not know on what basis your trainer commented that "full body routines are not useful".
What must be kept in mind is that, you will gain as long you work hard and keep your nutrition right, depending on the goals.
If you intend to follow a full body routine, you can find many online, I am sure.
Johnnie Walker - Keep Walking
"Move me out of the away so that someone else can lift": Ed Coan after injuring himself during a squat attempt of 970 lbs in 2002
POWERLIFTING - ABSOLUTE POWER