Google pixel not getting good Internet coverage upstairs

[GRice129]

My sky Internet box is downstairs by the front door, I have noticed over the last few months that when I am upstairs my phone is running slowly (things taking a while to load etc). I have tried forgetting and reconnecting to the WiFi, when I go to connect the signal on the WiFi box is showing as strong but when it connects to my phone it drops to fair. Appreciate this might be an issue with my phone but are there any other solutions I can try with the WiFi first?