That's interesting phenomenon. Which band you use 2.4 or 5 GHz ? Try the other one.

PCs in the LAN should not be affected by the SPI firewall - it's more about incoming connections from outside your LAN. Are you with the latest drivers for your LAN adapters. Have you tried with...

As a simple answer to your question I would say - I would use a dedicated firewall device before my router. Here some info about the router security.

Are you using the D-Link as WDS or as an AP connected by wire with AX50's LAN port ?

@Shuk Use that picture to re-configure the operation mode of the device: Then plug the ethernet cable coming from the provider to the LAN4 / WAN port of the VR600. Configure the Internet connection...

@J.J.A. Here you can browse the AX1500 web interface and see if that what's implemented suits you.