We have a bathroom in our basement that is connected to a grinder pump. The pump works. It has an air vent attached to it. Every time we flush the toilet our first floor smells like sewer gas. We have put a drain camera down the air vent from the roof and also from the pump going as far as we could. No blockages are seen but we were unable to get all the way through the whole system but only a small area is unseen. We have sealed the grinder pump very thoroughly. We have changed out all the toilet wax rings. We are at a loss. It only occurs when we flush the toilet in the basement. Suggestions? No other bathroom leads to this smell. HELP!

let the grinder pump run thru a cycle so you know the tank is empty. shut the power to the pump and flush the toilet a couple of times. see if you get any smell when water is used but the pump doesn't run.
then turn on the power and let the pump cycle.
i'm wondering if flushing the toilet or running the pump causes the smell.

We did that and had no smell at all…. So frustrating. We ran the sink to trip the pump. Unplugged it and flushed the toilet. No smell. Plugged pump back in and it ran but so smell.

"So frustrating" is not a strong enough term here..

i'm leaning towards a partial stoppage after the pump discharge ties into the main drain

How is the air vent attached? What does it look like? Can you smell sewer more strongly anywhere, like near the vent? Do you hear any bubbling?