It's a good thing you figured the problem before a final decision is made, hope your proactive action works out!
I believe Indeed ad is a good idea, but on top of that, it is even better if you could convince your former employer to write another reference letter that correctly describes your duties and other elements IRCC requires (don't forget ALL pages must have a company letter head), if somehow the HR can't/won't do it, maybe your direct supervisor/manager could help? Or at least a colleague who's still working there might be willing to clarify your duties back then? This is a matter of saving an application, def worths trying all possible ways.