power-line

As was already stated in the problem description, I would again check the positioning of the router. Are there any objects, big or small, in the way of the signal going to the second upper floor? Such objects could interfere with the signal strength. Think of the Wi-Fi signal as beams of light originating from your router that get reflected on each surface until they finally reach your PC or laptop and try to optimize it that way.
Furthermore, you can check your router’s settings for it’s channel settings. To prevent interference from your neighbours Wi-Fi sources the channel settings should be set to “Auto”. You can also try to set it manually, but I would recommend scanning all for Wi-Fis beforehand, with an app like Wifi-Analyzer.
There are also hardware options if you are willing to further invest into better Wi-Fi coverage. Simple Wi-Fi extenders like this one can be placed around your flat to amplify your signal by placing them at key points, e.g. near the stairs where signal might still be acceptable. Also, there are Power-line adapters that modulate your internet communication onto your power lines within your home. These can be hit and miss and depend on your home’s power lines.

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