Do you have any experience with OPC UA and the S7-1500's?
I have written my own OPC UA client driver for Android.
I do not have experience with S7-1500 but with other different OPC UA servers.
if it works with a server it should work with anyone since OPC UA is brand independent and 100% public documented.
If you want some OPC UA server for testing I have used "OPC UA ANSI C Demo Server" of unified-automation.com, it is a windows executable with simulated variables, some settings can be changed editing a XML file manually
Another possibility is to install Codesys porgramming software that also installs a PLC emulator (Soft-PLC) which has a fully functional OPC-UA server. The same happens when using a Raspberry PI as a PLC.
Also some time ago there was some public server available on the Internet but the ones I knew I think are now all down.
For me a main advantage of OPC UA is security.
Let's take an example, if a person silently connects a laptop to a switch in the industrial plant and the connected PLCs use non-secure protocols such as Modbus or Ethernet/IP or the old S7 discussed here, he can read and write variables in some PLC just knowing his IP and the address or name of the tag.
This can be completely avoided by using the OPC UA security features since it can be configured to only accept connections with a trusted certificate.
It's enough secure and encrypted to connect over the Internet without using a VPN