How to connect via RDP to machines in the domain?

Question

Active Directory

Hello. I'm starting to learn AD.
1) Delivered the server 19 + DNS + AD
2) Got 2 test PCs in the domain.
I set up RDP access in group policies (Permission to connect + set up a firewall)
As a result:
1) I can connect between PCs in the domain via RDP (using a domain administrator)
2) I can connect from my PC (outside the domain) to the AD server via RDP (using domain administrator)
3) I can NOT connect from my PC (outside the domain) to a PC in the domain.
Error: Invalid credentials. The credentials used to connect are invalid.
And I can log in via RDP if I use local PC accounts that are in the domain.
I'm sure I set something up wrong, but I don't know where to go, because I'm trying to log in under a domain administrator account, under which I can log in between PCs that are in the domain.
I didn't find anything on the Internet.

Answer 1

Everything turned out to be simple, corny, STUPID!
I wrote DOMAIN/Login
And you need DOMAIN \ Login
How simple it is. Thanks to those who responded
. I tried for 2 days to solve a problem that does not exist!