@ChrissTea Good day, 

Thank you for your detailed post, the negotiated speed is limited at 866Mbps because T9UH need 3*3 MIMO to achieve 1300Mbps (3* 433Mbps)

AX50 is 2x 2 Mimo -> 11ax: 2*1200Mbps @160Mhz ;  11ac : 2* 866Mps @160Mhz  ; 11ac: 2*433Mbps @80Mhz

Archer T9UH is 3x3 Mimo for spatialstreams   -> 11ac: 3*433Mbps @80Mhz

So 866Mbps is the highest link speed that T9UH could negotiate with AX50. 

@ChrissTea Hello, 

Sorry for the late reply.  The MIMO info are from our internal spec,  you may also find them on some third party websites.
The number of MIMO is decided by device hardware, some higher end ax router supports more MIMO, for example, AX6000/AX73 supports 4*4 MIMO & 160Mhz on 5Ghz.
Besides 802.11ax protocol and the number of mimo, higher bandwidth also help boost the speed. 

AX50 supports 160Mhz on 5Ghz while T9UH supports 80Mhz on 5Ghz. If your laptop could get 1.5Gb link speed, it may support 160Mhz& 2 MIMO on 5Ghz, so the speed is close to 1.7Gbps  2* 866Mps @160Mhz on 11ac.

If you want higher speed on T9UH, you could upgrade the router which supports more MIMO numbers on 5Ghz.

Or you could consider a 11ax wireless adapter which supports 160mhz to get faster speed on 5Ghz.