@egclassic 

My first inclination is that the AP router is only negotiating a 100 mbp Ethernet link with the main router .  This would explain the speeds you are experiencing.  If you made that cable yourself, try redoing each end, one at a time to see if it helps.  A "noisey" cable due to bad terminations, poor cable quality, OLD cable, RF interference, etc..., can cause Ethernet links to negotiate the lower speeds.  If it is a particularly long run, or is run near power cables, consider using shielded (STP) cable.  Just remember, you will need shielded RJ-45 connectors (metal ends).  If you are not familiar with how to properly use STP, terminate the cables and grounding, a little Google and YouTube can do the trick.  Many people use STP wrong by not ensuring the shielding is terminated to the metal casing and the jacks actually have a ground.  STP would be the overkill solution and most home / SOHO routers do not provide for shield grounding natively so you would have to do it in the wall drop.

All that said, their may be a setting in either/both routers that also may effect speed negotiations but I would look at the cable as most default to auto negotiate.  You can also try forcing the 1GB speed if settings allow.  I hope this helps.

  @egclassic 

I am having a few issues with the AX1800 as well. Specifically dealing with the internet port. I am currently in correspondence with an engineer from TP-Link. I am curious. Have you tried removing the cable from the internet port on the AX1800 and plugging it into one of the 4 Lan ports on the AP? Try this and share your results please. Thank you.