TTS is not mandatory, it is a just complete instruction that covers all scenarios.
Unfortunately, I don't have any One+ around so I can test. Maybe some system cleaner/antivirus/similar blocks access.
Or it is just up to OxygenOS ...

Another option remains - put the iGO folder content (move it using system file manager) in android/data/com.basarsoft.igonextgen.javaclient. Install from there.