No, sentinel island is where the Portal platform leads after dealing with Damian's goons- after you've dealt with Asmadius (the one passage that was blocked, is now unblocked.). you'll get a message when you step on the platform asking "do you want to go to Sentinel Island?" obv. select Yes and exit that area [if you're playing DKS, the two chests on either side have parts to an armor set], and the Island will introduce itself and tell you want to do.
Basically- you can't get to Sentinel Island from anywhere outside the Maxos Temple (not until you activate Waypoint Shrines on Sentinel Island). Go back down to the Crypt and hop on the teleport platform.
