These common issues however can be a network receptor hardware glitch of the iPhone 6s. To resolve these Apple recommends its iPhone 6s users some DIY (Do it yourself) fixes as a home remedy, such as:

• Toggle Airplane Mode on and off to reset the previous network settings and cache.
• Remove your sim card and reinstall it again. (requires a restart of the device)
• Set up the internet for your phone if you’re facing internet issues through the network for the first time. To set up go to Settings > Mobile Data > Mobile Data Network

then:

• Insert APN (Access Point Name) provided by your carrier 
• Insert Username (provided by your carrier)
• Insert Password (provided by your carrier)
• Save the settings by pressing the left arrow at the top.
• Make sure your Data connection is on after setting up the network before you access the internet.
• Install carrier update for your iPhone (Check-in Settings > General > About)
• Restart your iPhone
• Soft reset your iPhone

If none of the solutions work for your iPhone, you might need your phone to get checked by a professional technician. Here at ifixit bd, we have dedicated and experienced technicians who specialize in resolving your network issues in your iPhone 6s. ifixit bd has 4 service points which are located in points around Dhaka, Bangladesh.