I have a TP Link Router model TL-MR6400 and then to extend this wifi I have a Eircom D1000 modem, which is set up in Bridge mode. All my devices are able to access the internet using this setup(Windows laptop, Macbook, IPhones, printer, etc) except for the three android devices(A HTC 10, a HTC M8 and a Asus Tablet). All three of these are able to connect to both of the routers however when the Eircom D1000 modem is on then they cannot access the internet. When turned off my android devices are able to access the internet properly within three seconds, so I am confident that this is the cause of my problem. On all other wifi networks I have been on this problem does not arise.
Sometimes however these devices are able to access the internet, this can happen randomly throughout the day and lasts from 5 minutes to a few hours. Usually though the internet speed is much slower on them than on other devices.
This leads me to believe that the problem lies in the DHCP Server as when the devices are connected but not able to access the internet they are not appearing in the DHCP client list on the TP-Link. On the Eircom router the DHCP Server State is set to DHCP relay and DHCP Relay Server Address is set to the TP-Links IP.
Somethings I have tried already are changing the TP-Link's primary dns server to googles(18.104.22.168) as advised in this question and assigning a static IP to the devices as advised here. Both of these didn't help.