Some thoughts. These units are built similar to my SFG-1003. A common fault with function generators is user's overloading or damaging the output amplifier.
Do you get your set output frequency from the TTL out, ie.., DC- 4MHz? TTL out is a separate pathway from the 50 ohms output; if the square waves are stable and respond well to controls it means the DDS module is functioning. The TTL voltage output is fixed ~ 5V; the DC offset controls, and amplitude work on the 50 ohm output only.
If TTL works, then its likely the 50 ohm output amp. Trace the BNC jack to the board and follow it and it should lead you the amp.
If the TTL output is random or uncontrollable, its a much more involved troubleshoot.