I need to change my server IP but the seller says it's not possible! Why?

[julio]

If necessary, I can give you remote access via ssh, I have no problem.

 

[Richard G]

Please tell me what I should do to make sure my license is completely legal and has no problems.

In that case my answer in #4 applies. Go to https://www.directadmin.com and order an appropiate license for your needs.
You have a choice depending on the amount of domains you have to host.

Benefit of a new license directly from Directadmin is that all limits dissappear, so no MariaDB limit, no OS limit either and you have more options than with a legacy license.
Next to that, you can login and have your own license key.

They were never allowed to sell you a license seperately without a vps/server in a datacenter, so they are abusing the license.
You are not guilty as you bought in good faith and didnt know. But lucky you could use the license for that long.