I have a note 10+ on at&t and I haven't received the security update for April yet. Do I need to do anything particular.
No, all you need to do is wait until your Note 10+ tells you when the Update is available. I have the Unlocked Note 10+ and AT&T as well. Expect your phone to let you know when the One UI 2.1 update is available sometime this week however.
T-Mobile hasn't released it yet, either, or the One UI 2.1 update included with the April patch. Only Verizon so far, I've read.