I think "sales frontend" means employees who are assigned to talk to (potential) customers. In the case of a bank, that would be people at bank branches (for example, loan officers). "Sales backend", by contrast, would be all the people who handle a sales transaction after a customer has (at least tentatively) committed to a purchase. (Take a look at this web page for an example, though not in the banking industry, of the term "front-end" in a sales context.)
So the sentence "We need to improve our sales frontend in order to stay ahead in competition" could be translated as "We need to hire more people because we're not getting enough inquiries about our products." (I suppose it could also mean "We need to spend more money on advertising", but I suspect that has a separate jargon of its own.) Or "We need to train our people at the branch banks to do a better job of convincing customers to buy our products."