@RoryFan Since it's not clear what exactly is the root cause of your issue there's another thing you could do - downgrade the router FW to the previous release (v1.2.3) and test again. Another option...

@RoryFan How about your DHCP Pool, is it big enough to accomodate more that 15 devices: Make sure your LAN Subnet mask is 255.255.255.0 Any Access Control whitelisting, Parental Controls or...

@Moustacois Follow this guide to make sure that you have configured the Parental Controls feature properly. A correct system time setting is essential too.

@Graveyard I can't speculate if your router models are going to be updated with this feature any time soon, but the updated solution is from a month ago - here.

@Moustacois To be able to set the Parental Controls to work properly, you have to make sure that the laptop and mobile in questiion are not using a randomized MAC addresses - details. Keep in mind...

@Rick115 Reset the AX5400 Pro to its factory default settings by pressing the reset button for about 10 seconds. Then use the WPS button method to build the Easymesh - details.