Why isn't my Bluetooth printer appearing on iPhone but is appearing on Android and PC?
The thermal printer is not an AirPrint device - as such you cannot use it as a generic printer throughout the iOS system. I.e. this doesn't work like your regular inkjet or laser printer for desktop usage.

Instead the printer connects via Bluetooth and presents a "virtual serial port" to that is available to application developers. This means that the specific app you want to print for must be created to specifically support the DPP-350, or at least this particular type of printer language. 

This means that you need to find an app that specifically supports this printer before you can have any hope of printing with it from an iPhone.

In terms of printer language, this printer does not use AirPrint, PCL, PostScript or similar that would be typical of a normal printer. Instead it uses the old Epson ESC/P printing language, which were used for ordinary printers back in the day (i.e. in the 80s for example). Today it is only really used for smaller receipt printers and the like. They have relatively limited printing capabilities.