iPhone often not connecting to wifi networks

Unless my wifi signal is very strong (full bars) my iPhone will not connect to it. It will most often say my password is incorrect (i know it is correct) and continuously ask me to log in again, say it is unable to join the network, of will just show no networks available to join at all. This happens both at work and at home and my MacBook and other peoples iPhones seem to have no problem connecting.

I have an iPhone 6 with iOS 11.4.1.

This has happened in the past a little but lately it is consistent.

I have tried restarting my phone, forgetting the wifi network and rejoining, resetting network settings and setting a custom DNS.

Any tips would be much appreciated!