You may face a similar kind of problem with your iPhone 6 Plus. It usually happens after a faulty or uncompleted update where you might see that:

There’s no network bar available at the top of the display. Restarting or reinstalling the SIM card doesn’t help. There is always “Searching” shown on the network bar

There are no Cellular connections available for your phone. You can’t turn on, discover, or connect to available wireless Hotspot networks available near you through your iPhone 6 Plus

You can’t connect to your Bluetooth devices like Apple Watch, Bluetooth headphones, and stereo speakers through your phone. It appears that you cannot connect to anything wireless through your iPhone 6 Plus

If you are facing this issue go to Settings > General > About and see if you find information regarding IMEI, Bluetooth location, Modem Firmware, etc. If there is nothing there then you have an issue with the modem firmware. There are no DIY fixes available for this issue. You are going to be needed to take your iPhone 6 Plus to an expert repair technician to resolve the “No modem firmware” issue.