I’m a Wells Fargo customer (as well as having other bank accounts) and I like their online banking because it’s obvious they put a lot of thought into the customer experience.
Online, it’s not always easy to fix a problem as it happens. Most interactions are live but without life (human interaction). The next best thing is to offer a toll-free support number, and actually have someone there to answer the calls when they come in. After that, as far as email support, set an expectation and exceed it. i.e. post you respond within 2 business hours and get back to them in 1 OR call them back.