how do we call the Sunday that is after next Sunday?

In standard English, the Sunday after next Sunday is usually called the Sunday after next or the following Sunday. In some British usage, Sunday week can mean the Sunday a week after next Sunday, which is the same idea.

Simple way to say it

• Next Sunday = the nearest upcoming Sunday.
• The Sunday after next = the Sunday one week later.
• The following Sunday = also commonly used for that second Sunday.

If you want to avoid confusion

A very clear phrase is:

“two Sundays from now”

That removes ambiguity, especially because “next Sunday” can be interpreted differently by different speakers.

If you want, I can also explain the difference between next Sunday , this Sunday , and Sunday week in a tiny table.