Summon condition
"If you control a face-up "Ice Barrier" monster, you can Special Summon this card (from your hand). You cannot Special Summon any Level 5 or higher monsters the turn you use this effect."
is separate.
"You can Tribute this card to target 1 "Ice Barrier" monster in your Graveyard, except "Prior of the Ice Barrier"; Special Summon that target."
so. if you summon prior without using its summon condition, then you can summon a level 5 or higher.