This is a good question.
IMO, you don't need anything in that sentence though. And, I'm a non-native speaker. Though the question is not asked to us, I'd like to express what I think about the use of 'when' and 'while'. What is preferred by native speakers, they'd only answer.
Smartphones allow Internet access on the road - seems okay to me!
But then, if it's about putting while or when...
Both 'when' and 'while' indicate time-related actions. Both the words are quite often used in sentences with past progressive and simple past tenses. In such cases, while generally refers to a comparatively long action, on the other hand, when could be both a comparatively long or short action. Also, while has a clause with a simultaneous activity or ongoing activity.
In that sense, both seems okay to me. Smartphones allow Internet access while/when on the road.
Nevertheless, when you are going from one place to another place, your smartphone will get you the Internet access while on the road. And, when your friend asks whether smartphones allow access, you may simply nod and say that smartphones allow Internet access when on the road.