Can you tell me the exact model of Cyan toner you are using as there are two for this model a high and a low capacity one, and I also would like to know if you are using original brother branded toners or refill or aftermarket/non brother/"for use in toners". If you are using after market toners then the toner itself is probally faulty, these machines dont just sence new powder they look for a new chip to tell machine it has had its toner replaced, then the chip is blown so it cant reset the machine again, refillers have to put in a new chiop and powder if the chip they use fails then it doesn'r reset the machine, exactly like your problem.
If your are using brother branded original not refilled toners then we could have other issues so please then try the following. Please ensure the contacts on the drum and coresponding ones on the front door are clean. Gently tiny wipe metal wire contacts with a lint free cloth or a cotton bud with metho/alcohol, dont bend them they are delicate, see pic at end for location
Please also try to reset the toner manually, do this by openning front door and pressing clear/back button, a menu will appear, we want to select C-TNR-S or C-TNR-H depending on if you are using Cyan/TonerStandard or Cyan/Toner/High Capacity, so use arrows to go to correct one then press ok to select it, then a new menu appears, we want to select 1 asnd ok to reset, then it should just go back to cover open.
Also you could let me know what code comes up if you press menu * 2 8 6 and 4 real quick!!, if you do fast enough its tricky but then you should see maintence in display and it will beep, then select 82, and it will show eroor ?? let me know the code...also try to do a 80 it will print out a code sheet that I would normally read to identify problem, and finally in maintence mode select 99 to perform reset, you machine should go back to normal and out of service mode.
Hope this helps, or if required give me more detailed info and we will dig deeper.