Another WiFi question

I have a Brighthouse/Spectrum internet connection in my RV, paying extra so I can stream on my smart TV without using the park's poor signal. Using my router and cable modem. Tests at 100 mbps (advertised rate) on Ethernet, about 30 mbps on WiFi. Works fine for streaming and for my WiFi laptop. I test about 25 mbps on my IPhone 6, but trying to use it on my phone is almost useless. Disconnect from WiFi and the local LTE signal is blazing fast. Same with my wife's IPhone 7 (I always get her old one). Am I doing something wrong? Mainly for FB.

Try disabling the older 802.1 B/G protocols on the access point if they are still enabled and also be sure that you don't have a bunch of other networks nearby using the same frequencies.