Hi clarianesmaryjoy,
Please tell me your current firmware version, and mobile network provider.
Please try resetting your network. (Settings > Connections > More > Network settings reset.)
Please change to another SIM card by another network provider, and check if the problem still appears.
Please also put your SIM card into other phones and check if the problem appears.
Please show me a screenshot of the signal strength when the problem appears.
(Settings > System > About phone > Status > SIM status)
What kind of location does this happen? Please check in other locations as well.
Does this happen to specific apps or all the apps? What are the apps and their versions?
Does this message on the screenshot appear once you open the app?
If there are any screenshots or short video you can show us, it could help us understand the problem more clearly.
Thank you.