Please help. Pulling my hair out.

Thanks in advance for any assistance.  I've been having ongoing connection issues with my wifi, and after trying some tricks I read online I think I made it much worse.  The issue I've been having is intermittent connection throughout the day, regardless of what time (even at 2am). It'll go from ok, to very laggy, to non existent. I have a detached building about 90 feet from my router, so I purchased the extender and put it next to a window about 20 feet from the building (closest that I can get it).  When I go in the house and stand next to the router and connect to it I'll get upwards of 50mbps, when I stand next to the extender and connect to it I'll get around 20mbps. In the outside building I get 10-15mbps from the extender. The trick that I tried was to add aluminum foil behind the router bc it was having connection issues with the extender (orange light). I also changed the channel of my router from 11 to 6 since my next door neighbor is also on 6. Weirdly, the extender initially showed on channel 1 and the router on 11. Now they are both on 6, after I did a reset on the extender like I was told to. Now the extender connection, although showing 90% on the netgear app, is extremely slow. Up to 5mbps, while the router continues at 50mbps. I updated the firmware on the extender just now with no change in quality. The router is fine, and works great in the house, and our wired connection is greaet, but now my issue is I can't get a fast connection from the extender even though I have good connection.  I would think that if I'm getting 90% connection on the extender, the light showing the connection from the extender to the router is green, and the router is giving off 50 mbps, I shouldn't be having an issue witht he connection from the extender.  I mean, it's shows no connection 25% of the time, and when it's on I'm lucky to get 5mbps.  Any ideas?  Like I said, I've updated firmware, reset the extender, re set up the connection.   I don't know what else to do.