Unable to access ADSL modem settings and to ping my default gateway

I have small home network consisting of few devices. I used to have problems with redirecting malware on my Wi-Fi modem. After some effort the problem with malware seems to disappear. However, now I am unable to access settings menu of my modem. I tried from multiple devices, it is still impossible. Also I hard reseted my modem many times. It is possible to access menu for several hours after reset then it became inaccessible again. From cmd/ipconfig I defined my default gateway, pinging it gives 0 received packets. Searching for malware in all computers with different apps (AVG Free, JRT, AdwCleaner) also gives no result. What can cause this problem? How to solve it?