What platforms/services are the best to beta-test a mobile app puzzle game?

Based on my own experience building indie games I'd recommend these Reddit and Slack channels for beta testing:

Reddit:
https://www.reddit.com/r/AlphaAndBetaUsers
https://www.reddit.com/r/playmygame
https://www.reddit.com/r/indiegameswap/
https://www.reddit.com/r/freegames

Slack:
https://www.hamsterpad.com/chat/hashtaggaming
https://www.hamsterpad.com/chat/gamedevelopment
https://www.hamsterpad.com/chat/indie-game-devs

Also, just give it to your friends and family and you can record their interactions with it using something like: https://www.lookback.io/

Finally, some paid options include:
https://www.usertesting.com
http://ubertesters.com/
https://betafamily.com/

If you want to discuss how best to design customer interviews / questionnaires for your specific app, to assess what you need to change, what to keep, etc. feel free to send me a message,

best,

Lee