My Asus RT-ax86U router keeps dropping internet on wired and wireless

Either one could still be having issues.  Either intermittent signal or errors/signal problems on the modem, or networking on the ASUS perhaps.

For the modem if you want to take a look at it yourself, start here:  https://forums.xfinity.com/conversations/your-home-network/internet-troubleshooting-tips/602dae4ac5375f08cde52ea0  Bad signal in, power levels out of spec upstream/downstream/snr uncorrectable errors?  All not helpful.  Splitters, cabling, connectors?  All important.  Error logs can help too if it's got one to look at.  

IPV6 sometime doesn't work spectacularly well in my market with rebind/renew requests.  Page 83 on Asus:  https://dlcdnets.asus.com/pub/ASUS/wireless/RT-AC86U/E15867_RT-AC86U_UM_v4_WEB.pdf?model=RT-AC86U 

Might not even be enabled by default on ASUS.  You can see if it is too by going to someplace like test-ipv6.com and see if you are even passing out internal IPV6 to connected clients, but I'd check it out and you really ought to know what it's doing if you are trying to debug it.  Disabling for a test might tell you something.

Poke around in there a bit too at settings.  Maybe something set weird if it doesn't appear to be modem/signal related (that's usually the issue though).  Look at support on ASUS for perhaps a firmware, or other user issues too.