おにぎりはレストランで食べるものではないって英語でなんて言うの？

Rice balls are not served at restaurants!
"not served"
some restaurants do not sell rice balls, this is something we prepare at home or buy at a supermarket

Rice balls are not a common dish at a restaurant!
"not common" it is unusual to get at a certain place
"dish" a meal served at a restaurant

A recommendation is a suggestion of something you should try, therefore this statement is a good way of steering someone away from rice balls.

If something is done in a particular place or in a certain way then you would say it is 'usually' done in this way
So you would say 'Onigri isn't something you usually order at a restaurant'

Onigiri (rice balls) is rarely (if ever) found in restaurants...It's simply not on the Menu!
It is really an example of store-bought food... or a takeaway choice from one of the many convenience stores found everywhere in Japan:( 7/11 Lawson/ Family Mart.)
Or ... it is homemade and usually eaten as part of a bento box! Made to be eaten on the move as it were... it is very handy and easy to eat without chopsticks or utensils.

"Onigiri is usually something we make at home, not have in a restaurant." is a simple way to tell someone who is not familiar with rice balls that they are not on a restaurant menu most of the time. You can change 'something we make at home' to 'something we buy in a store' as well, or use both in your explanations to be very clear.
"Onigiri is a food made at home or bought from the store." is a great way to simply state the where you find the food.
Either of these examples can be used in a conversation when someone has asked about the place where the food can be found; you can also offer store names if that is helpful in being more specific.

Onigiri can only be store bought or homemade.
Store bought: means purchased in a store.
Homemade: Made from scratch at home.

We use made from scratch when a recipe is used to cook/bake something

Restaurants in Japan do not serve onigiri.
do not serve: means it's not available / not on the menu

When talking about onigiri (rice balls), you want to explain that onigiri isn't something you usually eat at a restaurant. It's usually bought at a store or you make it yourself.