Hello,
It’s difficult to say what is the reason of your problem we don’t have the link to your site. Please, provide me link to your site and temporary admin panel access to check the settings.
Did you do any changes in template files?
Can’t you update theme at all or just to version that is compatible with WC 2.6.x? Because the latest version that is compatible with WC 2.6.X is 3.10.1 and I could provide it to you if you need, it has a list of improvements and bug fixes.
Regards