I've been having this issue for months, maybe since I bought it and it's been bugging me more and more.
Whenever the phone has been locked for some time, it's as if it just cuts off the Wi-Fi. It shows that it's connected, but I'm never getting notifications on time and upon unlocking it takes up to a minute to open a website. Sometimes it even stops loading and just shows that there's no internet at all. Like I just received a notification for messenger that's ten minutes old. I feel as if it tends to do that even if I'm using the phone, but doing something that doesn't require a constant internet connection.
I almost want to blame my router, it's nearing a decade old, but no other devices seem to have this problem. Plus I've had that on public Wi-Fi networks at least once too.
Can anyone enlighten me about this? I've heard about some power saving shenanigans killing apps in the background, but this seems different? I'm still on 8.1, was hoping to wait until Pie before I complain about this, but it's taking too long and I want some answers.