Program universal directtv remote to onn tv

We have a 55” ONN Roku tv. We can change channels via the directtv box using the directtv remote,  but not adjust the volume on the tv. We have used the code search tool to no avail. We have tried codes for Durabrand and RCA. We have tried numerous manual codes. Can you help?

remote is RC66X

tv ONN model # 100021261