Slow 4G connection only, but only sometimes -- SOLVED

One thing to be aware of with wireless signals — and this applies to both the OP’s experience and what @tjweller just posted - is that it's important to understand how obstructions affect signal quality, AND the physics of how signals are obstructed. One of the most counterintuitive aspects is that if a signal has to pass through a solid object (a wall, window pane, etc.), the closer you get to that obstruction, the more of the signal it blocks.

The reason is simple trigonometry: When you pass through a solid object at an angle, it appears thinnest when you take an exactly perpendicular path (pass through it head-on). If you approach it at an angle, the farther from perpendicular you get, the thicker the object appears.

Consider this diagram:

The red arrow represents a perpendicular path, the thickness of the object is the same as triangle side 'a'. But if we take the blue path, the length of side 'c' represents the apparent thickness of the object (in terms of the wireless signal). The length of 'c' can be computed in terms of 'a' and the angle at 'x'. (See formulas. The last one just solves from Pythagoras: c² = a² + b².)

When x is 45°, c is roughly 1.4×a. When it’s 30°, c is 2×a. At 15°, c grows to nearly 4×a.

Now, a wireless signal is nothing like a straight arrow; that's a massive oversimplification. In truth, the properties of wifi signals are almost impossibly complicated, and calculating or predicting them is a science unto itself.

But the basic premise here still holds: A solid obstruction like a wall will obscure more of the signal being received at steep angles, when you’re right next to it.