UPDATE: I opened a ticket regarding this bug. Peplink was great to work with. They tested and confirmed that 8.5.0 has a bug that causes WAN DNS to send using standby WAN. They also stated that the bug was fixed in 8.5.1 (they shared test data that supports this).
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8.5.1 RC2 firmware can be downloaded here:
(per Peplink on NOV 21: The firmware 8.5.1 still in release candidate builds 2 (RC 2). The firmware passed the beta testing and now in release candidates version 2)
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Me, personally, I will wait for general release here:
(per Peplink on NOV 21: based on the info given by the Engineering team, we will have the 8.5.1 RC3 release in 1 to 2 days. If 8.5.1RC3 is working great, potential it can be the GA version in 1-2 weeks