Usually because you've moved to an area that isn't covered by LTE service or your wireless device last lost connection with the LTE signal. Sometimes you can force your phone to search for an LTE signal if you're certain that you're in a covered area. I've done this on many occasions with my iP6+ by toggling the airport mode button. Doing so usually restores my LTE signal if I'm indeed in a covered area.
man I am having this issue as we speak. I only left my house (which usually has no problem using 4G LTE except in stores & other privacy abuse ridden places which often is correctable by toggling the airplane mode as stated, performing a soft reset, or simply turning the device on and off) briefly but when I came back I noticed I was on 3G speeds and nothing I normally try is working to bring it back to 4G or 4g LTE! I performed a speed test and the results are so sad. I want to go faster, I'm in the process of buying a car and this is making it impossible to be an early bird. I get no worms 😞 my Scores are download 0.38 MBPS upload 0.02 Mbps ping 229 ms for Comcast server nearby....Wifi was 2.03 4.30 & 25 respectively. I'm not 100% sure what this means but I checked the numbers for accuracy so If you can't believe it (neither can I) I am open to rational reasons or better yet, solutions for our shared problem. I don't pay for 3G, I was told repeatedly that my device requires 4G data plan & that the old sloth speed was going to be obsolete. So why WHY when I haven't moved-havent traveled out of a coverage area, have been using googles data saver tool even though I don't have to - WHY CANT I LOAD ONE WEBPAGE or check my email.
You pay for data services. You pay the same for LTE as you do 3G. Neither is guaranteed everywhere. Your Comcast speed is WiFi and not Mobile Data network so Comcast has to handle that, but it could be that you were on a slow and congested network. Sorry if I read that wrong. Couldn't discern if you were speaking of Verizon speeds or Comcast WiFi speeds. my Scores are download 0.38 MBPS upload 0.02 Mbps ping 229 ms for Comcast server nearby....Wifi was 2.03 4.30 & 25 respectively.
If your phone only displays the 3G service, the enable LTE setting must have somehow gotten turned off. On an iPhone 6, to turn it back on, go to Settings > Cellular > Cellular Data Options > Enable LTE > Select Voice and Data. It should be a similar process for other types of phones.
I live in a very rural area and a lot of times the LTE won't have coverage but I'll still be in range of 3G towers so I can text and make a call but usually the speeds are too slow to even send an iMessage, use facebook messenger, or even load the most basic website. I keep wondering how this will affect my coverage come December 31, 2019 when they officially shut off all the 3G towers. Also, I travel internationally some. I'm wondering if my phone will still search for 3G networks when roaming internationally in an area that doesn't have LTE. I hate to see the 3G go, it's always a nice secondary as opposed to having no signal at all.