If you promote something and it doesn’t work there can be 2 reasons:
You promote it in the wrong way.
The product or offer you are promoting isn’t very attractive.
I think that there is a small chance that people will directly order after seeing the ad. I mean… You wake up in the morning, see the ad. But you are used to have dinner in the evening.
I have experience with promoting local gyms. It works great with FB ads, targetting just everybody living in that area. I promote: Coupon for 1 free week at the gym. The costs for each lead are less than $3.
You could try something similar with your hamburgers.
Another thing you can try is promoting with many IG accounts. If you make 20 accounts you can target 3k-4k people a day. Don’t focus on growing. You must use the moment they check out your profile. Make a link to an offer with a discount coupon that will be sent to their email. And once you have their email you can spam their email weekly with a new menu of your restaurant. You can send DM’s to new followers pointing to an interesting offer and the link in your bio. Only don’t make the mistake to put the same link in all bios. There are ways around that.