approx 2 wks ago we purchased 1 iphone 5s and 3 iphone 5c. we have been mostly unsuccesful with connecting to our home wifi. a few times it will prompt incorrect password and we can type it in and we can use wifi. mostly says unable to join our networ

we purchased 1 iphone 5s and 3 iphone 5s phones 2 wks ago. We have had no luck with connceting to our home wifi. when we try it usually says uanble to join our network. one in awhile you will get the message that its the incorrect password then when you enter the password it will connect. Is it possible our modem isnt strong enough for LTE? Is there a problem with our new phones? I know its very frustrating if you can't use wifi at home. ATT will have a field day with data overage charges. New phones are pretty much useless if can't use wifi at home.