The problem is mainly caused by your (poor) location relative to a cell site.
Of course, your phone has limited transmitter power because of its battery capacity
and the physical size of the semiconductors that do the job.
In other words, you can't increase the phone's transmit power by using a higher-capacity battery;
its electronics simply won't produce any more than they were designed to deliver.
Then there is the matter of received signal strength,
the power of the RF the phone gets from a cell site.
Transmitted power decreases according to an inverse square law,
which means that when the distance is doubled the power is reduced by 75%:
Four watts at a given distance becomes one watt at double the original distance.
A receiver can be made only so sensitive.
Even if there is some RF energy from a transmitter arriving at a given location,
it may not be strong enough to be usable.
So, when you are at a location that requires maximum signal strength to enable reliable performance
but the available energy is insufficient, you will experience the effects of weak signals.
In the case of cell phones and other digital radio transmissions, there is often a "cliff effect" ---
moving a few inches one way or another makes the difference between good function and no function.
However, there are cases in which digital signals act a little more like analog signals,
producing a "fringe" effect with variable results as you have experienced.
This is the same as with over-the-air digital TV reception that "breaks up"
due to marginal signal strength.
Cell signals can also be blocked by objects such as buildings.
The exact effect of this depends on several factors.
The only no-cost thing you can do yourself is move closer to the nearest cell site
and/or move so that you have better exposure to the site.
There are cell phone boosters,
but be Very careful about selecting one.
None that will actually work will seem cheap,
and many of those that aren't cheap may be in some way inappropriate for your situation.