I’ve hit a wall with radiant echoes/restored coffer keys and shards. Not sure what to do.

Let me start by saying that I’m very unfamiliar with end game content in WoW. I’ve never hit max level in any of the expansions. So I’m not totally comfortable with weeklies, dailies, world quests, etc. That being said, at the moment I feel like I’m doing all that I can do to obtain these currencies, but I feel like I’ve hit a wall. I used all 4 of the coffer keys that the game guides you to and have also used all the echoes that I looted from the bountiful chests. I am now at 0 keys, 0 echoes, and 99 key fragments. So I can’t open the chests anymore for more echoes so I can get more fragments. I know there’s an answer to this problem, I just can’t seem to find one after some serious googling. I am truly digging for an answer to this, but there doesn’t seem to be one out there, so I’m just hoping I can get a response to get some clarity on what I’m surly missing. Thanks!

Edit: Answered! Thank you all so much for your prompt and respectful help!!