As a point of interest when was the last time your re-sync'ed, and have you ever left it off for 10-30 minutes? Though, that might not be necessary. I think on the ASUS you can close the PPP session without disconnecting the DSL? Try that if it can be done.
What I'm after at the moment is ending the PPP session for a few minutes, and possibly picking up different routing when you re-establish one. That could very well be what happened when they sorted out the initial problem, that you got onto a different entry point into the gateway(s).
We may be chasing a red herring with the wifi aspect. Though if your good experiences were on 2.4GHz, go back to it on the ASUS. Don't make both changes simultaneously, as if things do improve we won't know which cracked it.
Kindness isn't going to cure the world of all its awfulness but it's a good place to begin. Daisy Ridley.
My broadband basic info/help site - www.robertos.me.uk.
Domains, site and mail hosting - Tsohost
Connection - AAISP Home::1 80/20. Sync 65618/13914Kbps @ 600m. BQMs - IPv4