unable to activate my iphone after updating to ios 11.

since sep 2017 i am trying to activate my iphone 6 after updating to ios 11. i tried every thing restored my device several times, tried with different computers, different sim cards, different wifi networks, itunes.....etc but my device is giving same error every time.

"activation server cannot be reached"

when i am trying to activate my phone with itunes then error message appear that

" Iphone could not be activate because activation information could not be obtained from device.....disconnect your device, make sure sim card is inserted correctly and if you are using sim pin, tap unlock to enter and then reconnect the your phone"

i have good cellular signal but i have this issue. please help me for activate my iphone.