Thats weird, because always when i set private dns in phone settings or in rethink/invizible it always shows me in dns test providers i set in those settings, not tor ones, like its still over tor
If I recall correctly tor works differently on android and it’s less anonymous than the desktop counterpart. I did not test the dns leak on android.
Edit: I checked on mobile and tor browser for android uses his own dns too.
I think your rethink/invizible setup is the issue here.
Thats why i use desktop when i can, but sometimes i still need to use phone, thats why i got idea for firewall which i use in second space with tor, but in other post people were telling to use only tor for more anonimity, if it would be better to use private dns tho separate from tor i would use dns crypt with relays or odoh (split or after exit node), but i seen its adviced to use only tor 
I think you’re over complicating things.
Keep it simple, setup a private dns on your android settings to enjoy filtering options and use tor browser to navigate anonymously when needed.
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I just tested it on Android and there’s no leak in Tor with a private DNS. So the problem is in your settings. Just follow the instructions I gave you and it’ll definitely work.
Working good from v0.5.5t.