You're on the right track with Portal and The Witness! If you enjoyed those, you might want to check out "Go Vacation." It's not strictly a puzzle game, but it includes tons of fun mini-games that challenge you without overwhelming you. Another great option is "Unravel," which combines beautifully crafted puzzles with a touching story, and it's definitely beginner-friendly. Have you tried "The Room" series? They're pretty much the gold standard for mobile puzzle games, and the atmosphere is super immersive. What kind of puzzles do you enjoy the most, logic, environmental, or something else?