Unfortunately, Bungie decided to smush a new leveling system into D2 with Edge of Fate, instead of just making a separate D3... and didnt bother to update anything from before Edge of Fate to that new system ... so now half the game, including campaigns, old raids, dungeons, gambit, anything from the planet screen is the "old system" that wont give you any of the updated gear or high-level rewards. The only activities you can do to level up, are all found on one singular page that looks like a Hulu menu, called "The Portal". You'll want to open up this tab & select an activity, then choose the highest difficulty it allows (Master, Ultimate, etc) and proceed to add various "modifiers" to the activity, until it shows you a desirable reward package on the bottom right of the screen. You will be graded on your performance & will need to earn the grade shown in order to earn the projected reward. These graded activities are the only way to earn higher level gear... I wish I were kidding.

You can't level up past 300 through the Destination tab activities (legacy raids, dungeons, sttike / crucible / gambit playlists, etc). You can only level up to 550 by doing The Portal tab activities (Edge of Fate, Renegades, etc).