How do I stop my Apple TV from buffering?

As stated your issue is likely a weak home network. Most users when told this indicate they have a high speed connection. However the speed of the connection while important is only one facet of having strong network. Buffering is normally caused by poor coverage of the existing network. For example if you have a home that is 1500 sq ft. or larger and you have many Wi-Fi devices (phones, TVs, streaming devices, computers, tablets, thermostats, lights, locks, etc....) all of these thing compete for your signal. In addition if you have devices that are far away from your router and are separate by walls and halls then your signal isn't going to be strong in all parts of the house. In order to get a strong signal it's wise to look into a Mesh network that will cover your whole home.

If you are unfamiliar with Mesh networking, please look for some articles that explain them. For example here is just one:

https://support.google.com/wifi/answer/7182746?hl=en

To reduce buffering, get better internet and/or home network.

(No setting on the device can eliminate this.)

The downside of extenders (whether it is an old Airport Express or some other type) is they are ½flexed meaning their signal is immediately cut in ½ this is why they are generally a poor solution. They were all we had prior to Mesh networking but now that Mesh networking is in the mainstream and is affordable it is a much better solution.

Get an Ethernet cable into your Apple TV (instead of Wifi which looks weak at your place).

OR, get an Apple Airport Express (wifi extender), this is how to use it https://youtu.be/atxGhC_TecY