Didn't find exactly same topic. There is a simillar topic but seems not read by TP-Link staff. Hope this post can draw some attention.
I just switched o Deco M9 Plus and noticed that the router frequently switches 3 out of 4 iphones and 1 Fire TV from 5GHz to 2.4GHz. All other computers, the 4th iphone, ipad, echo dot, etc, seems stable at 5GHz.
On the affected iphones, if I do a network reset and connect to the router again, it initially connects to 5GHz but then switches to 2.4GHz at some (random) point.
I read that this might be feature to best utilize the bandwidth. However, sometimes even I start media streaming but it doesn't switch back to 5GHz.
And for other devices, even they don't use much internet for a long time (ipads, one iphone, echo dot, etc, in sleep mode), they are still assigned to 5GHz, why the router frequently change the connection to these few iphone and fire tv? even if this is a feature, it does not do a consistent job.
using a guest network to split 5G and 2.4G is not a good solution as I have 2.4G devices such as printer.
I tried a Netgear Orbi AC2200 and had no such problem at all. Haven't found any permanent solution yet. I am very sure if this is the firmware issue or hardware issue (this unit only). but strong suggest TP-link put high priority on those features that are really needed by customers.