@Ryoga I have the same adapter, it will connect on 2 channels, My iPhone can connect on 3.  They are still updating the drivers for these so maybe it will connect on more channels in the future.

  @Ryoga It's pretty much based on signal strength, I have another system with an Intel wifi7 chips.  It sits right above the router and uses 5,6. The one with the tp-link adapter is farther away and has more walls in between.

  @Ryoga if 6ghz at 320mhz channel width is in range and used, the adapter will use this only instead of trying to work across multiple frequencies since this already gives the maximum link-rate that the adapter support (5.8 Gbps). This also has much less possibility of interference since there are not as many devices transmitting on the 6ghz band as 2.4 or 5ghz.

Wifi 7 is still young and new, only certain adapters / routers are designed to use all 3 frequencies at once if they are in range.

as for your 1.5gbit internal speed test, what program / file were you using for transferring between the two computers and what was the wifi link rate at the time of transfer on each of the computers?

If you are transferring a single file stored on an NVMe drive (that has a sequential max read/write speed of at least 3GBps), then depending on the program and protocol used for transfer (if it uses multiple threads on your CPU unlike traditional SMB through file explorer) it should be higher than 1.5Gbps. CPU power settings in Windows and the model of CPU can affect this of course as well.

  @Ryoga This is normal operation. The MediaTek MT7927 chip installed in the TBE550E is a DBDC (Dual Band Dual Concurrent) chip that supports 2.4G+5G or 2.4G+6G STR mode. Therefore, 2.4G (40MHz) + 6G (320MHz) = 688Mbps + 5764Mbps = 6452Mbps, which is displayed as approximately 6.5Gbps.

The 2.4G + 5G + 6G MLO you mentioned is called TBTC (Tri band Tri concurrent), and as far as I know, there is no client that supports this on Windows, and the only one that supports this is the MT6653 on the MediaTek Dimensity 9400.