Just saw your post. If you're still looking for a Modbus RTU (RS323 or RS485) to Bluetooth converter to connect to mobiles, IOTize have a product called TapNPass that might help you: https://www.iotize.com/tapnpass-line.html
It includes an app for Android and iOS devices that provides you either a simple terminal or an interface for selecting and reading/writing data to Modbus registers.
If you need to create an interface dedicated to a specific task, you can also configure the TapNPass and create your own HMI apps for Android or iOS devices.
You can do it over Wi-Fi instead of over Bluetooth.
Wi-Fi ModBus gateways are available on the market.
It does not have to be a wireless modem in the environment. You can connect to Wi-Fi network with "Access Point" mode of Wi-Fi Modbus Gateways and view the data of your device via mobile application.
Hello, Quick follow up on this thread. IoTize are releasing an automatic app generator that works for Android and iOS. The idea is to eliminate the need to do any coding to create an app. This soution is based on the TapNPass Modbus to wireless adapters. You simply connect the TapNPass to a port, Configure it, and Generate the apps from the configuration environment.
This video shows how TapNPass works in a test implementation on an equipment that support Modbus RTU: