My phone's data connection is too slow while under the roof. I have checked if there was any problem with service provider but

Solution
The only people who can address that is the carrier/service provider. They control the speed. Most providers have a cap on their speeds and/or data limits. So if yours is like some after you hit a certain amount you either have no connection at all, or you have a serious drop in speed. Usually it goes from 3G or 4G (depending on what you have) down to 2G.

Also, the location can be a large factor, in a few ways. If there are many other electronics around, either yours or your neighbors, that can interfere. Also they type of building and where in the building you are, as well as your location in comparison to the tower. Some of those you can change (like turning off electronics you aren't using if they are part of the problem. And some...
The only people who can address that is the carrier/service provider. They control the speed. Most providers have a cap on their speeds and/or data limits. So if yours is like some after you hit a certain amount you either have no connection at all, or you have a serious drop in speed. Usually it goes from 3G or 4G (depending on what you have) down to 2G.

Also, the location can be a large factor, in a few ways. If there are many other electronics around, either yours or your neighbors, that can interfere. Also they type of building and where in the building you are, as well as your location in comparison to the tower. Some of those you can change (like turning off electronics you aren't using if they are part of the problem. And some you can't effect, like where you are compared to the tower. A good way to check that last one is to go to your carrier's/service provider's site and look at the map of their service area. Be sure to enter your address and then zoom in, a lot, to get a good look at your area. While it won't show when zoomed out on most carrier's/service provider's maps, it will show when zoomed in if you are in a less than perfect connection area.

If none of these helps, and you have a WiFi internet connection where you are, you may want to consider turning off data (when you are there) in your phone's settings and turning on WiFi for your internet connection.
 
Solution