We have it attached in the usual position next to the mirror. It never gives any problem.
For a long time we only had one Telepass and we swapped it arround as required. With the plan we have you can have two cars on the one Telepass but you can change the car (plate number) at any time using the app. Since then they have offered a second Telepass at no cost so, with the fleet reduced to two, we can keep one in each car. Only one is enabled for international use so we can swap that as required. The other is enabled only for use in Italy.
Some years ago, before the international Telepass was available in Italy, we had a VINCI télépéage which also worked for Spain and Portugal. That was attached with Velcro to the sunvisor and worked quite well there.