Low water level in both toilets

It could be a siphoning problem. Do you live high up on a hill, a block of flats or in a three storey house. Where is the soil pipe terminating and is there an air admittance valve, if it terminates in the roof space, or is it open and terminated above the roof externally? If there is an air admittance valve fitted to the soil pipe in the roof space, try removing it temporarily and see if it resolves it. If so replace the valve. If it is an open soil pipe then maybe try fitting an air admittance valve on an accessible waste pipe upstairs or an anti vac trap to the bathroom basin. I hope this helps

I attended a similar problem a few months back and it was the air admittance valve sticking inside the loft at the top of the soil vent pipe. See if you can find where the soil vent pipe terminates and if there is an air admittance valve, temporarily remove this and flush the toilets and see if the water level goes back to normal. If it does then a new air admittance valve will need to be fitted

Did the company camera the drain line. There maybe a leak in the line.

Best solution for not wasting time is installing booster pump above stop cock cost effective and saving money and plenty of water pressure any time