You never edit server settings in phpmyadmin.
They are set by the server via a template as usual.
NB If you read the wiki you would see mysql57 is already EOL.
Maria 10.x might be a better bet.
Either way, to see where there are settings we can check the config entry eg "mysql57-mysqld" and you can search for it like this:
grep -rn mysql57 /etc/e-smith/templates
I don't have this installed so can't say specifically what to modify.