Loss of powers up hill
 

So I have a 2007 mazda 3, it is automatic with tiptronic, (I always just drive it in the automatic function. )I am loosing power when I go up hills, my car would not go past 42km/hr and the RPMs were going up to 5. No matter what I tried it would not gain speed on the hills, as soon as I’m on flat roads it would gain speed no problem and seem to run fine. Does anyone know what the issue could be?
called a mechanic they said it’s transmission bring it to a transmission shop, Brought it to a transmission shop and they said bring it to a regular mechanic it’s not getting enough horsepower. Can anyone help or has had similar issues?