why not to try open mod menu on left side , and apk target on right side using mt manager , rename the class.dex mod menu apk to what ever inside game apk was , like if in side target apk has class.dex class1.dex then just give it name to class2.dex on mod menuapk , then copy class2.dex from mod menu to the target apk
image :
left panel was mod menu , right panel was game target
rename classes.dex to classes2.dex on mod menu to match count from classes.dex on target game ( see above picture in right panel )
after rename was sucsessfull then touch and hold classes2.dex on mod menu apk until menu appear , then click on + Add ->
click ok and wait for a bit
as you can see it added the classes2.dex to target game apk ,
after it was added just added lib folder from mod menu apk to target , then decompile target apk and added permission and service to androidmanifest.xml and add smali string to com.unity3d.player.unityplayeractivity , then recompile using apk tool.