“I use a Lumia 650.I don't have a plus sign on the keyboard!”
Not sure whether all phones are similar but when you get the keyboard up try hitting the “123” button and there maybe a button with the #+= symbols and hit that and the []{}#%^*+=_\|~€$¥•.,?!’ Symbols should be there.
Behind the first symbol page
It’s small so just scour the first symbol page