@423tommy’s correct. Your impulse is going to be to want the inside of the oven to look perfect and uniform. But resist.
It sounds like you have not yet applied anything over the exterior of the firebrick. If you have complete voids in the mortar (by which I mean you can shine a flashlight into your oven through a gap from the outside), you could try a shallow patch from the outside with very stiff mortar. Wet the brick first, and don’t try to fill it all the way through. The arch construction will support it when you get into the upper courses of brick, and gravity will keep it from seeping in the lower courses.
The idea, and I must emphasize this, is to avoid the impulse to fill it all the way through. Just don’t. All you want to do is to seal the exterior from hot gases.
And again, if your exterior mortar work is presenting a solid surface, just leave it alone.
One of the least popular topping combinations for pizza is anything that includes mortar crunch. Ben & Jerry’s tried it as an ice cream variety, once. It didn’t work for them, either.