After turning on merge of css and js files in
System > Configuration > Developer and clearing the cache, files are not merged
You may try this with SSH once. You can please execute below commands:
php bin/magento config:set dev/js/merge_files 1 php bin/magento config:set dev/css/merge_css_files 1 php bin/magento setup:upgrade php bin/magento setup:di:compile php bin/magento setup:static-content:deploy -f
I have already used these commands in many projects. This should be working fine for you as well.
Let me know if you have any more queries.