Waste Water Treatmet problem (Bio-Cycle system)

I buit my house around 10 years ago and a certain Waste Water Treatment company based in Munster installed the system.

I did not sign any maintenance contract with this company. Over the years I've noticed that the system alarms out annually without any audible or visual alarm. I only realize I have an issue when my toilets start to back up and a stink comes from the tank around the manhole.

Then, of course I ring the installers, they come out, "service" the unit and I have to add that they reset a password on a mitsubishi controller. they charge me 200 euros and walk away.

This happens every year. I don't mind servicing the unit but I feel that this company are forcing me to use their services even though I have not signed up to any contract. I've asked them for the 4 digit password but they will not give it to me. I would like to use someone locally to do this for me.

My toilets are currently starting to back up as my system is not working. I have to add that they usually add a comment in every year to my receipt that there is a slow flow, blockage etc. But I can 100% confirm that this isn't true. The flow etc is perfect up to the tank.

I would appreciate if anyone has seen or heard of anything like this before??