Apologies in advance, I'm not particularly techie, but I've done my best to resolve this and have hit a brick wall.

I'm paying for a 1 gb connection but realised I was only getting 500 mb over wifi so I spoke to my provider who explained that was the limit of their routers so I bought the AX53 in the hope it would resolve it. 

I set everything up and the speeds were the same, around 500-600, but found this useful post in the community https://community.tp-link.com/us/home/forum/topic/25483 and set it up identically. I managed to get DL speeds of around 790 and upload speeds of 980ish.

I've noticed things were slower this morning so checked and am now only getting 90mb/s, and have called my provider to see if there's an issue in the area but they've said everything is fine but they can't help further as I'm using a third party router now. 

I've tried different channels and channel widths but nothing gets it above 90mb/s. Does anyone know what I can do to resolve this or how to figure out what's up?