Your PHP execution time limit is still 90 seconds, you can see that here under the server tab. That means whatever you've done it did not work, please contact your hosting support and ask how to increase site's PHP execution time limit effectively, they might have limiting that on their end.
Another thing, is the PHP memory limit you can see that here as well, even though 256M is the minimum requirement, please increase that to 512M as I can see you'll going to use plenty of plugins, and your site's backend is really slow, increasing PHP memory limit will help on that. You can follow the guide here on how to increase the PHP memory limit. Again this method might not work because some hosting does not allow their users to do this on their own, if that is the case please contact your hosting support and ask how you can increase PHP memory limit effectively.
If the issue persists, please contact your hosting support and ask to deactivate the ModSecurity, then check Theme Options to see if that resolves the issue. If it does, since deactivating ModSecurity is not recommended, request to enable it back and unblock/whitelist only the
Let us know how it goes,