Simplest method is to open the template admin and then select save as copy option. Name this so you recognise it as fixed. Go to Advanced tab and select 1200 Fixed option. Now assign this template to the menu item where you do not want responsiveness.
If this resolves your problem then please edit your first post in this thread by adding SOLVED at the start of the title and then select the green tick icon. This lets the moderators know that it can be moved to the solved section.
Thanks for your cooperation
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