As described, the WiFi signal is poor in some areas of your home. WiFi signals are essentially line-of-sight, but will easily penetrate lightweight construction materials such plasterboard/drywall and timber. Signals are heavily attenuated by high density materials (e.g., stone, concrete) or blocked entirely by metals. Obstructions, especially those close to the router, will create zones of low/poor signal.
You may improve signal coverage by moving your router to a different location, preferably towards the centre of your home. So,e experimentation may be required to locate a better position for the router. Alternatively, you might consider installing wireless repeaters; for example, BT market their own “disks” as part of their Whole Home WiFi solution:
https://shop.bt.com/products/bt-add-on-disc-for-whole-home-wi-fi-091073-CVVT.html
Other solutions are available - such as Powerline repeater/access-points that use the AC-mains wiring to efficiently distribute your network around the home, but still using WiFi to connect between the repeaters and your devices. These are available from many different manufacturers.
A further popular choice is to invest in a Mesh network router and repeaters. These are effectively a turbocharged high performance version of WiFi repeaters. Again, these are available from multiple manufacturers.