Almost everything is related to how strong a signal you have and most those issues are how far you are from the cell tower and what is in between. I assume you are talking inside your house which means the walls of the house which do a great job of eating he signals.
There is not much you can do about this. Almost all forms of amplifiers are illegal to use and many are scams. It is in the grey area legally if you can put different antenna on equipment but it is not a option for a cell phone anyway. Most mobile routers and hotspots also do not have a way to connect antenna because of the rules the FCC has.
Now this depends on why you are asking. If you already have a internet connection and you just want to fix poor cell coverage in your house then you can actually put in your own private cell tower. This has a couple names but commonly called a micro cell. It is a device that creates a vpn tunnel back to the cell company over your internet connection. It is pretty low power cell tower that you restrict to only your devices. This is mostly used to fix dropped calls. Mobile data will work but it is kinda silly when you could connect via wifi to the home internet. In addition it actually uses mobile data out of your cap even though it is running on your internet connection.