You should have reported this to the police.
And you should do that now though it will be harder to get them to act.
Regardless of whether this it the teller's fault or not the crime here is fraud and you have been the victim. This is actually a well known fraud and if you google secret shopper now you will find out that you are one of many, many victims unfortunately.
If you are correct and the teller erred then you are not liable for the funds as you would not have taken them out of the bank if you knew the cheque was not valid.
I cannot know if the teller should have been able to tell or not.
So here is what you do.
First you go to head office of the bank and you complain.
They will investigate and then if they don't agree with you, you file a complaint with their bank ombudsman.
If their bank ombudsman doesn't agree with you then you to to the governmental bank ombudsman who is impartial. But you cannot go to this ombudsman until you follow the route that I have explained.
If you go into any branch of your bank there will be a brochure on how to complain. All the contact information you need, for each step I've set out, will be found there.
That should ultimately work and if the teller/bank erred this will be determined.