Why i can't set my network to LTE only *#*#4636#*#*

[Android Central Question]

i dial *#*#4636#*#* next phone information next preferred network set to LTE. I was able to do it before a month or two. But suddenly it set to GSM only i can change it to WCDMA and unknown, LTE/GSM PRL auto. But when i choose LTE only it goes to GSM only instead LTE only help hopefully someone understand and answer it.