Hello,
Thank you very much for choosing our theme and for contacting us.
I hope you have made the full backup of your site before the theme change. Please restore your site from the backup.
Then follow these steps:
Deactivate all the plugin on the site,
Install the Basel theme as per instruction: https://xtemos.com/docs/basel/installation-process-2/
Install and activate all the plugins from Appearance > Install the plugins.
Finally, activate the third parties plugins one by one checking the site to detect which one causes the problem.
Do not activate the plugins related to the previous theme (theme unique builders and others)
Best Regards