If your connection is from LCO, then you have to contact the LCO guy who configured your device for the default gateway address and password for login because
TP-Link does not use default username/password for its device instead it asks you to create a new password before the first login or after a reset. I think the default gateway address might be 192.168.1.1 or 192,168.0.1. I don't have a tp-link
ONT but I do have a tp-link
router connected directly to a
Huawei ONT and I also use zoiper5.
After going through your ONT's
specification, I have found that it does support adding a static route and also you can enable Sip ALG under NAT Forwarding. So I think you can configure your ONT to make zoiper5 work.