1) ask the model of the phone
2) search network settings reset steps

Situation: 
What shows on the device screen when you have the problem?
Do you get any error messages?
What were you doing with the device when this started happening?
What devices are having the issue? Are other devices not having the issue?
For Dual SIM with eSIM devices, which line is having the problem?


Duration: 
When did you first notice it?
How often does it happen?
Can you repeat the same steps and always get the same problem? Or is it sporadic?


Location: 
Where do you normally experience these issues? Home, work, school?
How many bars of signal do you have? Do the signal bars fluctuate?
Do you see 3G, 4G, or LTE on your screen?
Are you inside or outside?