Ok so mine is connected to the bottom, however, the spout it's attached to is totally sealed up. It looks like it melted. And no fluid will pass through. So at about 4 bars on the gauge it starts to lose fluid around that hose and then runs hot. Gets up to the last bar flashing.
Crud will accumulate at the bottom.
The reservoir should have one hose out the top running somewhere safe to vent under the vehicle. Normally this breathes air. If disaster strikes it can blow steam.
Generally there is a space in the neck where radiator cap installs which is routed to the bottom of the reservoir. The radiator cap has two seals, one spring loaded at the bottom. Another larger in diameter at the top. When pressure overcomes the spring hot steam vents out the hose to the reservoir.
When engine cools coolant contracts it produces a vacuum. In the center of the spring loaded plunger of radiator cap is a smaller metal plate which is very lightly spring loaded. Vacuum opens this plate and draws liquid from the reservoir. You can open it with your fingernail to verify it is not stuck.
For the system to work the top seal of the radiator cap must be good else the vacuum will suck air. The hose to reservoir must not collapse under suction. And the fitting at the reservoir must not be blocked.
I think you have two related problems. A blocked reservoir will not overheat your engine. When coolant is changed a working reservoir is necessary to burp air out of the engine. Every heat cycle vents to reservoir and whatever air is in the system tends to be at top of radiator so the first thing out is air. Cooling draws liquid back. If there is any more air the next heat cycle will vent air out, liquid in. Repeats until all air is gone. Then heating vents hot liquid coolant.
Air in the cooling system results in lower cooling capacity. But not huge losses in cooling as seems to be your problem. Not unless your engine coolant level is significantly low. Start by checking quantity at the radiator cap. And fix the reservoir.