Give mysql privileges to user and change sql_mode?

Question

SSH

Good afternoon.
Essence of the question. There is a large database that is imported to a new server (Ubuntu 18, VestaCP).
On the previous server, the configuration was done for me, because. and to this day I am not competent in this matter.

It is necessary to change the sql_mode of this table so that errors like "default value" or "only_full_group_by" do not appear. I don't have superuser rights.

What I tried:
1. In PHPMYADMIN itself, in the "variables" section, I found sql mode, but the values ​​were not saved there, because I didn't have superuser rights.
2. I created a root user for mysql via SSH with a password identical to the user and its database, but when I log in to PMA under the root login and this password, I get a refusal, although the password is correct, because
3. Tried in mysql> mode, connecting via SSH to give privileges to an existing user with the command

GRANT ALL PRIVILEGES ON database_name.* TO 'database_user'@'localhost';

but got a response

ERROR 2006 (HY000): MySQL server has gone away
No connection. Trying to reconnect...
Connection id:    1040
Current database: *** NONE ***

4. Increased max_allowed_packet to 1000M in the VESTA panel for mysql, but I also got the error from point 3.

Answer 1

The config has been corrected.
But here comes the next problem. When the server (not mysql service) is restarted, sql mode returns to its previous position.