Start with the allowance
A limited-data plan sets an included amount. Read what happens when it is used: extra purchases, reduced speed or loss of data access can be different possibilities. The handset's usage meter may use a different date range from the provider's billing cycle.
Background updates, cloud backups and tethered laptops can consume data without a long browsing session. Check per-app usage and update settings, especially after replacing a device.
Distinguish a cap from congestion
A fixed speed policy applies under its stated conditions even when the network is quiet. Prioritization affects how traffic is treated during contention. Congestion means demand is competing for finite capacity at a location or part of the network.
A plan threshold can change priority or speed without removing all data access. Ask which rule applies to phone data and which applies to hotspot. A video-specific policy may affect streaming differently from other tasks.
Collect evidence before changing plans
Compare the same task in two locations and at two times. Turn Wi-Fi off briefly to test cellular service, then restore it. Check whether only one app, only uploads or only hotspot is affected.
Keep an observation log without private credentials. A more generous allowance may solve a quota problem, but it will not necessarily repair poor indoor reception, a damaged device or a failing application server.