A phone can show full bars and still have no internet. Bars measure the radio link to the tower; data needs a separate session with the operator's network, and that session can fail on its own.
The common causes are administrative rather than technical: a monthly data cap reached, a data saver rule, a billing hold, or roaming disabled while travelling.
Configuration matters too. The access point name tells the phone how to reach the operator's data network, and a wrong or blanked entry after a SIM swap stops data while calls keep working.
Testing is straightforward. Toggle airplane mode, then compare a data-only app against a call. If calls work and data does not, the fault is in the data session, not the signal.