With a new modem and router, it could be the configuration of the new one that may differ from the old that is causing the problem.
Can you post your current configuration settings for the wireless connection (you can take a picture with the built-in 'snipping tool' and post it on here) and also the type of laptop of which the problem is occurring to.
Can you post your current configuration settings for the wireless connection (you can take a picture with the built-in 'snipping tool' and post it on here) and also the type of laptop of which the problem is occurring to.