[AC68U] USB modem doesn't load every site when connected to the router

I have an ASUS AC68U router and I'm currently using Tomato by Shibby (Tomato Firmware 1.28.0000 -3.0-132 K26ARM USB AIO-64K) but I've used AsusWrt-Merlin before to connect my HUAWEI E3276s-150 USB modem (4G dongle) to the router and provide Internet access around my network.

Setting this up is really easy with Tomato. I just plug the USB dongle to the USB 2.0 port, and configure the Internet connection like this:



Since I took that...

[AC68U] USB modem doesn't load every site when connected to the router