@varnicous 

Possible reason would be either the PC or Router is not seeing what it saw before, so it can't connect correctly.

Usually, when install a NEW ROUTER, the Router and Modem has to be off. Then power up the Modem and when up fully, then Power up the Router.

Now, with the router IF the ethernet cable are connect AND the Router had already been on, DISCONNECT the Ethernet Cable before powering on, and when fully up, reconnect the Ethernet cable. Should then bo OK.

Alternatively, at this point, power off the Router and then on might work too. To be sure, disconnect the Ethernet cable before you power the router on.

Still have a problem, go to the PC and FORGET that connection and then it should create a new connection.

  @IrvSp 

I tried all of that before even posting.

also to note, I hooked my old nighthawk X6 7900 back up and instantly got 450 up and down.

  @varnicous 

I do have an AX55 but not using it now. I had no such problem.

It could be QOS is set somewhere limiting speed.

Check the Tether App or Parental controls if enabled.

Might try RESETTING the router to 'shipped' state and starting over again too  See https://www.tp-link.com/us/support/faq/497/https://www.tp-link.com/us/support/faq/497/ 

I assume all PORTs on the AX55 have the same speed as well? Did you try a different cable (I am pretty sure if the same cable was used before it wouldn't be the case)/

All else fails, contact Support,