Freedom Mobile in Toronto where I live was like that. Sure you can get Band 66 everywhere in the coverage area, but since I was using a "legacy" device (which is what we call pre Band 66, my 6S Plus, taken from the term "legacy streetcars"), it was only available in certain areas. I would get Band 7 LTE at home, but Band 4 UMTS at work. This fact alone clued me in at least for work, that LTE upgrades in that area had not been completed. Now that I have my 8 Plus, I get LTE everywhere, but sometimes it'll drop to 3G if the signal is stronger for that than LTE to keep the connection alive. I might be wrong here, so please correct me.
It does take time for LTE upgrades to be completed. According to the Freedom Mobile website, LTE upgrades for the Greater Toronto Area should be finished in the Spring of this year, but for home using my 6S Plus, I'd been on Band 7 LTE since December 2017. Since I now use an 8 Plus, I get Band 66 LTE everywhere. Sometimes I'll get on Band 7 LTE, and if it's not looking good, it'll bounce me to 3G.