I’d wager almost all sellers use both laptop /desktop + mobile. It would be a bit weird from Fiverr to offer a mobile app but not allow you to use both laptop and mobile.
You can use as many devices as you want as long as you use them with only 1 account.
I suppose in theory you could get unlucky with a dynamic IP. Where I live, almost everyone has dynamic IPs, I never had a problem with it, but in countries with many users and even more accounts than users, I guess chances are higher to get an IP someone else with Fiverr account used before. Probably Fiverr checks for more than IP, thus, though. (all my assumptions, I don’t know)