surprised about all the replies and none of them mentioning what, I think, was the first and most "native" app for tvheadend since like forever :
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it does have in-app purchases, but not to remove ads (as far as i can tell it doesn't have ads), but rather to enhance it's features (but they're all about adding/editing recording timers etc ... watching channels and browsing the EPG is, if i'm not mistaken, fully enabled in the free version).
keep in mind that if you want to watch from outside your network (port forwarding etc), TVHclient uses the 9982 (!) port to communicate with the tvheadend server, so you'll have to expose both ports (normally 9981 is enough for watching/streaming).
That was my thought as well and I wanted to buy the Hauppauge WinTV Dual TV-Tuner, but I never got to it because of the financial situation. If you try out some other tuner, please let me know how it goes! :)
Seems your TV has a low spec to run Kodi smoothly. I had a similar issue with Nexus Player. I found SPMC app (a fork of older Kodi) requires less resource and run better as TVH client. Give it a try. https://play.google.com/store/apps/details?id=com.semperpax.spmc16&hl=en_US
I tried other TVH clients but the free versions have limited function.
Check out tv dream player. I use it to connect to my pi tvheadend server age works like a superbly l, so much faster and smoother than the plex / hdhr option I used to have.
https://play.google.com/store/apps/details?id=de.cyberdream.dreamepg.tvh.tv.player
There are versions for Amazon Firestick and Shield. Both work flawlessly once setup, including live recordings. Have to pay for app but well worth it imo