If not compatible, set the Deco as AP only.

First, u need to get the old router to work again. Resetting will restore it to factory default. U will then need to key back the username, password, VLANs back.

Refer here, https://kevron2u.com/tm-unifi-maxis-fibre-v...ting-at-router/

Once done, factory reset the Deco and use the app to set it up again. This time, as AP only.

Before doing this, make sure the stock router is working and connected.