Try to remove the SIM card and then put it back again.
Then try to optimize the range of the mobile network, see this Apple article:
If you see No Service or Searching on your iPhone or iPad
If you see No Service or Searching on your iPhone or iPad - Apple Support
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If you see 'invalid SIM' or 'no SIM' on your iPhone or iPad
If you see 'invalid SIM' or 'no SIM' on your iPhone or iPad - Apple Support
Manually update your carrier settings on your iPhone or iPad
Manually update your carrier settings on your iPhone or iPad - Apple Support
sims card unlocked hooked too unlock iphone 5 but phone says sims cards locked please power cycle if problem persist to contact provider
Have you contact your carrier / ISP?