Hi BruceWayne007, I followed the video and got everything working. So I don't know what to tell you.
You've probably already seen this, but just in case... There's WireGuard documentation on the DDWRT forum:
DD-WRT Forum :: View topic - WireGuard guides and documentation
If you're still having trouble, I would try posting your question on the DDWRT forum. They are generally very helpful.
Also, have you flashed your router with a recent version of DDWRT? I think DevbaseMedia was using a build from around Jan. 1, 2021. I downloaded my firmware here: ftp:/ftp.dd-wrt.com/betas/2021/
I haven't tried setting up a WireGuard client on my iPhone. I don't know what to tell there.
Good luck.