Have you still got the original remote that came supplied with the white now TV box ?
If so, you should be able to go into the now TV box menu settings and under Network select the new SSID network name on your new supplied Router, then somewhere on your new Router there should be a sticky label with the password to use.
Normally when you factory reset or switch ISP with a different Router you need the remote that came supplied with the NowTV box to get it back to a state where you can use the Roku remote App on your smart phone.
If you haven't got another nowtv infra red style remote (black or white) lying around, then if you have still got the old router you could try this.
Personaly if it was me, the white nowtv box is a bit long in the tooth nowadays and doesn't support all the third party Apps available like the newer nowtv devices and I would be looking at a replacement to one of the newer version NowTV streaming devices (unless your television doesn't support HDMI and your white nowtv box is using the red, white and yellow analogue connections on your older television set).