BGW 320-500 some iPhone apps fail to send or receive data.

Hello. I have 1Gbps fiber with a BGW 320-500 residential gateway. On my BGW 320-500 I have the following settings:

WiFi: Both bands disabled

Home Network -> IPv6: All settings "On" and router Advertisement MTU is 1500

Firewall -> Packet Filter: Disabled

Firewall -> IP Passthrough -> Allocation Mode: IP Passthrough

Firewall -> IP Passthrough -> Passthrough Mode: DHCPS-fixed

Firewall -> IP Passthrough -> Passthrough Fixed MAC Address: The MAC address of my 3rd party router.

All other settings on the RG are default

My third party router is a D-Link DIR-X4860. I have the following settings on my D-Link router:

Internet -> IPv4:

• Designated as Dynamic IP (DHCP),
• Primary DNS server set as 1.1.1.1
• Secondary DNS server set as 1.0.0.1
• MTU is 1500

Internet -> IPv6

• Designated as "Auto Configuration (SLAAC/DHCPv6)
• DNS type is "Use the following DNS address" with Primary set to 2606:4700:4700::1111 and secondary set to 2606:4700:4700::1001
• LAN IPv6 Address Settings has a toggle for Enable DHCP-PD which is enabled
• Address Autoconfiguration Settings include:
  • Enable Automatic IPv6 Address Assignment: true
  • Enable Autoamatic DHCP-PD in LAN: True
  • Autoconfiguration Type: SLAAC+Stateless DHCP
  • Router Advertisment Lifetime: 30 minutes

Firewall is disabled on the router

WiFi 2.4 Ghz and 5ghz are both enabled and use the same SSID

Some ports are open my local devices that need them. 

Overall the internet seems to work great. But on my iPhone 12 and my wife's iPhone 12, some apps will be slow to load data and only specific apps. Running speed tests will show wifi speeds at 300Mbps or more. Specifically, if people send images or videos or sometimes rich text content via Facebook Messenger, it will not load the content for quite some time or until I disable wifi and re-enable it (assuming its loading it over the cellular network). 

I feel like I have also seen the occasional slowness over Reddit as well. And in some occasions, even just sending text will not send. It seems like restarting the equipment can resolve it, but only temporally. I would really like to find a way to permanently fix this, as it is very frustrating. Does anyone have any experience with this or any ideas?