wi-fi is turned off when locking screen 5s

Hello! I have a small problem. I have the iphone 5s (RFB, 16Gb). After switching on the lock screen, after 1-3 minutes of Wi-Fi turned off, when you turn on Wi-Fi phones are automatically connected back (4G Wi-Fi router) on the home Wi-Fi router is no such problem nablyudaetsya.Samostoyatelny search in the phone settings and 4G Wi-Fi router does not give results. Tell me how to fix this problem. Sincerely, Roman (Russia).