Completely unable to figure this out

Hi all

I have come to this subreddit because I am at my wits end. I feel like everything I know is wrong all because of one little 8 person network.

This is a new client of ours. The router is a Mikrotik hAP Lite (v6.45.1), primary WAN is fibre with a PPPoE dial-up.

Router is DHCP & DNS server, 8.8.8.8 is set as the secondary DNS server by DHCP. Mikrotik has static DNS servers (8.8.8.8 & 1.1.1.1) and responds to remote queries.

Wifi is provided by UBNT Unifi AP AC LR, the network cable goes from the injector into the Mikrotik (to rule out issues with the switch). AP is running the latest firmware, and is given a static IP.

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Clients get DHCP address, they can ping the router but cannot get out to the internet. If they are on cable they can.

Previous WiFi setup used two TP-Link Extenders setup as APs and plugged into ethernet cable.

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Clients will stay connected for a few hours before they are unable to get DNS queries resolved.

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At first I thought it was the WiFi, so we installed the Unifi AP, then I thought it was the switch so we plugged in the AP into a port on the Mikrotik. Earlier today I was battling with a users laptop - disabled Windows firewall, and it started working again. 2 hours later... client phones and no one on the WiFi can get internet access. I even completely wiped and reconfigured the Mikrotik today thinking I didn't do something correctly.

I honestly do not know what I have missed so I have come to this subreddit in the hopes that you might see something I don't.