iPhone 6S+ keeps dropping WiFi :(

Hi all, I have had this issue for quite some time now. Every few times a day, my WiFi will stop working. The WiFi icon still appears in the top corner like I am connected, however, it doesn't work. The only way to fix this issue is to quickly toggle the WiFi off and back on again, and it starts working normally again. This happens at work and at home, so it can't be network related.

When it isn't working, I've noticed that if I go into Settings and then WiFi, the only network I can find is the one that I am connected to. My router has 4 different networks, and only the one I am connected to is visible when things aren't working.

iOS 11.2.1, iPhone 6S Plus

Does anyone have any idea what is causing this? Thanks