You can try with an extension that changes the user agent of your browser; if you use the one from either firefox or chrome for android it should work. Usually its the only thing checked by sites when identifing browsers for non telemetry reasons; if the problem persists I’m not sure what it could be
Then it may be something that mull is blocking to prevent fingerprinting that duolingo doesn’t like, I don’t know how to help in this case other than installing something like fennec in a work profile with shelter; sorry.