Get a WiFi monitoring app for your phone then you can walk around the house and check out the signal strength of your and your neighbours' WiFi.
That will give you a good idea of whether the issue is related to your neighbours using WiFi at night.
Also the phone is likely to be dual band anyway and you can see if your neighbours are using the 5Ghz bandwidth and if so, how congested it is.
If the phone is Android, then I recommend Wifi Analyzer by farproc from the Play Store.
Ex - Plusnet Customer (2009 - 2023) now with BT
