Hmm, after upgrading to 2.5.7 i also ran into troubles using Stellarmate app (on a Samsung S6 Tablet).
I am using a dual-IP setup, one is on the local WiFi, the other one is the Stellarmate WifFi network, where the Raspi acts as a hotspot for the private 10.250.250.0//24 subnet.
I was away from the home network, so the 192.168.0.x network could not be reached, but i wasn't able to create a new device config for 10.250.250.1 (SM hotspot) while being in the same subnet with both devices (Phone and Raspi), because the app got stuck in the optical train setup dialogue. At home, the 192.168.0.x network/node was available again in the app, but i could not connect because of the stuck dialogue. Then i deleted all app data and cleared app cache, so i was able to create 10.250.250.1 config in the app again, but it acted awkward on connect. Nothing worked, i had to shut down the Rasapi using ssh.

I am quite confident that there is a bug which will get resolved with the successful addressing of Christian T's bug and issuing a fix.
If there's a workaround, please post here, in the meantime i'm using Stellarmate OS directly via VNC, which works well as soon as you got the hang of the slightly different EKOS dialogues.

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