@johntyson
What is the model number of your TP-Link router?
Below is a quotation related to the 3rd party firmware:
To Use Third Party Firmware In TP-Link Products
Some official firmware of TP-Link products can be replaced by the third party firmware such as DD-WRT. TP-Link is not obligated to provide any maintenance or support for it, and does not guarantee the performance and stability of third party firmware. Damage to the product as a result of using third party firmware will void the product's warranty.
Back to your case, we can try to troubleshoot and verify whether if we can figure it out, otherwise, I'm afraid that you may ask for help from others.
1. Please check whether your computer can get internetw ith it connected to the ethernet port of the ONT. Click here to check the IP and gateway.
2. If so, you can connect computer back to the router by wired cable and reconfigure it, besides please clone the MAC address of the computer.
3. If not, you need contact the tech support of the ONT to check the network connection.
May it help and have a good day.