Router settings

I have an Asus RT-AC1900p router connected to an Arris cable modem for my 
network.
I have 2 desktop PC's ( one running Win11Prox64 and the other Win10Prox64), 
a Win10Prox64 laptop, and numerous phones, tablets, and Amazon fire sticks 
sharing this network.
When I first set up the AC1900, it was "sharing" bandwidth pretty well. I 
could download a large file on one device, and all the others would still 
have internet functions- I could browse, run on-line games, etc. Now, after 
only a month of using this thing, it won't hardly "share" at all. I download 
a large file, and try to connect another device, it gives me a "no 
connection" error. I have checked my settings, and even restored the 
settings to an earlier save, and it still doesn't work like it did.
AFAIK, nothing has been changed on the router or the cable modem. The 
individual NIC settings on each device haven't been changed. It's just 
puzzling to me why it should work so well for a while, then not.

Any suggestions on what I should check on the router, modem, or any other 
settings will be appreciated. Thanks for reading this!
-- 

SC Tom