I don't have a specific solution but some things for you to consider for testing purposes....
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Although they both claim to be 802.11ac / 866Mbps...it looks like the zyxel has less antenna diversity than the 4A. If that were the case then there would be hiccups. (I've seen that between ar750s and C7, both use the same chip but the ar750s can only hit 433Mbps at 5Ghz vs the C7 which can do 1300Mbps - I meshed them and they didn't like it).
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easy answer is to repurpose the zyxel and buy another 4A. Or vice versa. Keep the nodes homogenous / same models and on the same chip family for best results usually. Repurpose the zyxel as an AP or edge router.
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I couldn't get pictures of the PCB inside the zyxel so it is hard to say what antenna layout it is using. The pictures of the 4A looks like it has real antennas and not some PCB or hybrid which is what I think the zyxel is using.
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collect system (and kernel?) logs when the mesh starts acting up. Post them on here. Strip out any identifiable info such as mac addresses.
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post your UCI configs for network and wireless for the 4A and the Zyxel for comparison purposes. Strip out personally identifying information.
I apologize in advance if alot of this is redundant. It sounds like you tested alot!
HTH
Crazy idea...
- Disclaimer: for testing purposes try rolling back the zyxel's OpenWrt version? Probably not advisable due to security patches. See below:
Updated:
- the Zyxel does have 2x2 antennas like the Xiaomi 4A, so it must be either IPQ4018 vs MT7621 conflict or settings.