Air lock in toilet system

[Sarahc]

All 3 toilets won't refill as there is an air lock in our system. They are fed from a water harvester in the garden. The water travels to a tank in the loft and then filters to the toilets. We've had it happen 3 times in the last 2 years. A plumber comes and attaches a hose to the sink tap and attaches it to something in the cistern, what does it attach to in the cistern please?? Thank you