The radio is not the whole path
The phone connects through local network infrastructure and then onward to an application or server. A busy local network, a plan rule, an overloaded destination or an app problem can all affect the result. More bars do not remove those possibilities.
Wi-Fi can add another path entirely. First establish whether the failing task uses cellular data or a local wireless network. Temporarily separating the two can prevent a broadband fault being misreported as a mobile outage.
Look for a repeatable pattern
Compare the same task at another time and location. Test more than one application. If only hotspot slows, inspect tethering conditions. If only video changes, review video policy and the app's settings. If all tasks fail everywhere, inspect the line's active state and provisioning.
A speed test can be useful, but note its time, connection and server context. Repeated large tests also use data; they are not the only way to show that a normal task fails.
Change the thing the evidence points to
A plan with a different data allowance may help a threshold problem. It will not necessarily fix poor indoor reception or an application server. Give official support the smallest reproducible example and the checks already completed before considering more disruptive resets.