DIAGNOSIS The printer's toner counter hasn't been reset. The machine tracks drum usage separately from the physical cartridge.
FIX Reset the drum counter. Press the 'Go' button three times. Wait for the machine to cycle. The 'Toner Life End' message should clear. Use only the genuine Brother TN-730 cartridge for this reset to work reliably.
IF THAT DIDN'T WORK The drum unit itself might actually be at end of life. Open the front cover and inspect the green tab on the drum unit. If it's fully to the left, the drum is expended and must be replaced (Brother DR-730 unit).
HEADS UP Never use non-genuine or 'chipped' cartridges on this model. They can lock the counter and require a full main board replacement to fix.