ZURICH—UBS posted its highest quarterly profit since 2015 on Tuesday, as robust trading activity by the world’s ultra-wealthy led to a 23 percent surge in fee income. The surprisingly strong results follow double-digit percent gains for net profit in each of the past four quarters and come as Switzerland’s largest bank announced new plans for its digital push including a new advisory service for affluent U.S. clients. Third-quarter net profit came in at $2.279 billion, easily surpassing a median estimate of a 24 percent slump to $1.596 billion from a poll of 23 analysts compiled by the bank. “We continue to perform well above the financial targets that we have given you. And while we do that, we continue to be focused on driving growth on the top and the bottom line,” Chief Executive Ralph Hamers said on a call with analysts. The bank’s flagship wealth management business posted its …