Issues with internet connection using bridge mode on Telstra Smart Modem Gen 2 & Asus RT-AC68U

Context

My Telstra service worked well over wifi using the default setup (using the Smart Modem as the modem and router). However instead, I want to use it in bridge mode to take advantage of my Asus router's features.

Setup

I've used the following steps to try to enable bridge mode on the Telstra Smart Modem 2 (listing out since this isn't documented anywhere by Telstra or the manufacturer):

1. Disable wifi using the physical button on the rear panel
2. In the Telstra modem admin GUI at 192.168.0.1, Go To Advanced -> Internet -> WAN Services -> Toggle 'bridge enabled' to off. Confirm and wait for the progress bar and for the modem to reboot. After reboot, the admin GUI is no longer accessible (as expected in bridge mode).
3. Connected Telstra modem's LAN port to Asus router's WAN port.
4. Setup the Asus RT-AC68U router's SSID and leaving other router settings out of the box. This uses Asuswrt-Merlin firmware, version 384.14_2.

Unfortunately there isn't a technical user manual available for the Telstra modem - the only manual is the very simplistic device setup guide.

Issue

After the above, the internet connection works well using LAN ports, and the wifi mostly works on iPhone, but with some issues. The primary issue is that Whatsapp cannot connect (says "Connecting..." with never-ending progress pinwheel) which means messages aren't received until I'm off the wifi network. Similarly, connections to Lifx and Apollo apps are patchy, however these are less important.

Any suggestions out there? I'm hopeful the above gives some clue to more experienced users on what settings I should look into tweaking.