Spotify and Signal run flawlessly on it. I recommend Spotify Lite because it requires fewer resources and is easier to use on the small screen. Threema will probably also work.

Even What's up doesn't work for me...

I do not know

The operating system of the Beafon SL860 is (for me): Android 8.1.0 According to the operating instructions of the Beafon SL860, you can install additional apps via the AppStore. I don't know whether Signal and Telegram are included. It also says: "... Please note that not all apps are compatible with the Beafon SL860 ..." I have never installed apps from the AppStore and therefore cannot say how stable such apps are on the Beafon SL860. Probably some run well and others do not.

Plus is Spotify also possible?