1, go to Settings - General, in the management of storage space software usage list, see whether the iOS11 installation package downloaded, if there is delete it.
2, check whether it is full and close to the limit, with little remaining 3G storage space.
3, clean up all background, shut down and restart and try again.
3, go to Settings - General - Restore, restore the network Settings and try again.
If necessary, restart the modem and router, and download the speed measurement tool to detect the network.