@newtpuser Hi, if you only have problems with your Xiaomi A3 should probably start troubleshooting there, do a google search and you will find many with similar problems on Xiaomi A3. I even had a Xiaomi phone that didn't work as well on 5GHZ bands as I had to send back to the store.
you can test if it works better on either 2.4GHz or 5GHz band, mine worked best on 2.4MHz but this band is not very fast.

If You start with a SSID with 2.4GHZ like this. if it's not bether change to only 5GHz

@newtpuser are in the USA?

My home network is versy similar to yours (Fios Gigabit fiber w/Verizon Router), two AP's (both EAP225 V3), and Actiontec MOCA 2.0 Bonded.  One EAP is Ethernet the other is MOCA.  I don't use a POE switch, I use the POE injectors that came with the AP's.

Sometimes my MOCA will drop off leaving clients "stranded" on the one AP.  I also used to use EAP245s.  Since switching to EAP225 w/MESH and installing some UPS'es this is no longer an issue.  The one EAP prefers Ethernet (MOCA) but if the MOCA flakes out it fails back to 5.8GHz MESH.  I run about 40 clients at any given time.  6 laptops, several phones, ipads, etc.  One Roku (rest are hardwired) and a variety of IoT, NEST cams, etc.

Start by disabling band steering and use separate, different, SSID's for your 2.4 and 5.8GHz networks.  Also disable airtime fairness (if enabled).  Join your Xiaomi A3 to the 5.8GHz (assumng it supports it).

Check your channels and channel interferece with your neighbors.  I find I need to use channels 3 & 9 for my 2.4GHz (20 MHz only) because my neighbors are slamming channels 1, 6, and 11.  I run 80MHz wide on 5.8GHz manually selecting one low and one high channel.  Enable fast roaming on the OC200.

Try lowering your transmit powers to medium all around.  But, as others have said, if it's just that one device likely its a wonky driver or something.  Make sure it's fully updated.

I find no real advantage to using my older EAP245's (V1) despite the 3x3 since all of my clients are only 2x2 anyway.  Generally, I see 350-450 Mbit/sec DL & UL over wifi when I'm in the same room with the EAP.  This drops to ~150-250 Mbit/s in other rooms.

-Jonathan

@JSchnee21 @shberge