I began using WLED with iPhone 6 a few years ago and it worked just fine until it did not in 2021. iPhone 6 use ios 12.5.5. Although the WLED UI opens and the color wheel works, no effect can be chosen in the UI. My iPhone SE (15.4.1) works just fine. However, I prefer to keep the iPhone 6 with the WLED systems and use the iPhone SE for communication. Is there a WLED version that will work with the iPhone 6 or is that a lost cause?
I have an Iphone 6 model MG572LL/A running ios 12.5.5 and like you I could only do on and off and select colors but I cant select effects and its very slow , this is also the case when using safari or chrome to access the wled instance ( tested on normal wled 13.1 and SR 13.0-B6 )
The only MCU that I was able to control with iphone 6 wled app was flashed with Blazoncek fork .
You could give that a try .