PPPoE encapsulates all traffic into a tunnel, which is almost always compressed and frequently encrypted. Between the two PPP peers the packets would be essentially un-QoS-able, an all-or-nothing proposition. The end-points can apply QoS to the VOIP, but it would still be limited by the scheduling of the PPPoE tunnel's packets across the radio link.
I solved this applying DSCP code 46 or 48 and this is tagged throuth the same scenario + 1 ptp link direct to main gateway/internet router by appliying in prerouting masquerade this class of traffic acting in this MKtik router.