what should I do if my mobile could not detect wifi signal in a distance of 5=6 steps away from the modem?

If your mobile can detect other wifi networks at different locations your mobile seems fine.
If it is just with a specific modem wifi problem please check the following:
1 - Your modem's wifi antenna is loose.
See if you cas fasten it.
2 - Your mobile has a wifi compatibility issue.
Try change your wifi security mode from WPA to WEB
3 - Your mobile's wifi is faulty
Either reset your mobile or return to supplier.

Hope this helps
Grant