Hi @g12863,
As with MT4, the trailing stop for MT5 “is executed in the trading platform rather than on the server”. That means you must place your trailing stop on the desktop platform and keep the platform running at all times for the trailing stop to execute.
One solution is to run your desktop MT5 platform on a virtual private server (VPS). You can then connect to the VPS from your phone using a remote desktop app like RD Client.