A phone's data warning can help reduce unexpected usage, but it is not a real-time reading of your SimFuse balance.
Android
Look in Settings > Network & internet > SIMs, or on Samsung under Connections > Data usage. Choose the correct line and look for a data warning, limit or billing-cycle setting. Availability and names vary by device.
A hard limit can stop mobile data even when the plan still has data left. Check this setting if the connection unexpectedly stops.
iPhone and iPad
Use the cellular usage counter and, where helpful, Low Data Mode. The standard settings do not provide the same configurable hard data cap as some Android phones.
Reduce background usage
Download maps on Wi-Fi, limit cellular app updates and cloud backups, and check hotspot usage. Check the plan in SimFuse regularly rather than relying only on a warning or push notification. See "Check how much data you have left".