Welcome to TPG Community!
I was able to locate your account using your community details and learned that you have a TPG Mobile SIM account with us. I've seen that you've already contacted our support team and verified that the SIM is working with your phones.
If it's still not working with your Telstra dongle, you might need to contact the manufacturer to provide further troubleshooting.
I have a Telstra Wingle (4G+, WiFi, USB Dongle).
I have use it seamlessly with the VR1600V when the NBN connection goes down. It has a Telstra SIM card in it. Last week I went to the Telstra shop to get it unlocked (because their pricing is too high) and I was told directly that it could not be unlocked because it has Telstra Branding. And that was the end of that.
I've since found out that Aldi SIMs are on the Telstra Network and that the Aldi SIM should work in the Telstra Wingle. I'll let you know next Month if it does.
Did Telstra give you the Network unlock code or the forgotten password unlock code?
I'm sorry to say, but your dongle is most likely still "locked" to the Telstra Network and that is why you cannot connect