sshd: Connection reset by IP [preauth]

Initial Post here (closed as not relevant to ServerFault)

I have a raspberry pi with only Key based authentication enabled. When I try to ssh to it using the local IP, it works. But when I try to connect it using a noip domain (dynamic dns), I get Connection reset by <IP>: [preauth] instantly.

Things to know:

1. I've changed the default port from 22 to another in /etc/ssh/sshd_config and yes the new port has been forwarded.
2. I've restarted the pi and the sshd service.
3. I've tried changing up the private/public key pair.
4. I've tried tailing the /var/log/auth.log file when I try to connect via ddns and all it gives me is the Connection reset [preauth] error
5. I have a static IP

What works: If I disconnect from my ISP and ssh via my phone hotspot, it works.

What I think: It's the static IP issue? I don't have much knowledge on the matter but as a lay-person I can understand that there might be an issue with the RSA fingerprint or something?

Any help would be appreciated! Thanks.

Logs (sensitive info redacted):

=> ssh -vi ~/.ssh/key <USER>@<DDNS> -p <PORT> OpenSSH_8.1p1, LibreSSL 2.7.3 debug1: Reading configuration data /Users/kunalnagar/.ssh/config debug1: Reading configuration data /etc/ssh/ssh_config debug1: /etc/ssh/ssh_config line 47: Applying options for * debug1: Connecting to <DDNS> port <PORT>. debug1: Connection established. debug1: identity file key type 0 debug1: identity file key-cert type -1 debug1: Local version string SSH-2.0-OpenSSH_8.1 debug1: Remote protocol version 2.0, remote software version OpenSSH_7.9p1 Raspbian-10+deb10u2 debug1: match: OpenSSH_7.9p1 Raspbian-10+deb10u2 pat OpenSSH* compat 0x04000000 debug1: Authenticating to <DDNS:PORT> as '<USER>' debug1: SSH2_MSG_KEXINIT sent Connection reset by <STATIC_IP> port xxxx 

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