Strange loss-of-internet connection issue (possibly router/modem-related)

This might be a long shot... But I'm stumped. (sorry for the long post)

I've been battling issues with my internet connection (Spectrum cable).
At random, it loses the internet...
Rebooting (unplugging/plugging back in) the router & modem restores it.
Sometimes it stays working for many hours, other times it keeps going out again & again after only a minute or two.
Occasionally it might reconnect if left alone for long enough (3-10 minutes), but still just as likely to go out again at some point afterward.
When it goes out, nothing can connect to the outside internet (not with wi-fi, not ethernet). LAN still works... I can log into the router no problem, which it gives me this screen:



And also the modem itself (RF connection is fine):



I have additional modem screenshots if anyone needs (such as Downstream & Upstream Bonded Channels and other stuff all beyond my knowledge).


The weirdest part of all is, this only happens when my father is here (happens if he's using a device at the moment or not). Rarely does it happen when he is not here or is out of town (so apparently it's one of his devices causing a conflict or something).

He is not computer-savvy, he just uses his laptop (an IdeaPad w/Win-10) to browse the web & email (anytime there's a problem, he comes asking me to fix it).
We both also have Samsung phones, but the Wi-Fi on them is normally shut off (we rarely use them for internet at the house since we have PCs, they are more for when on the road or out of town).


I have tried & tried searching the 'net for solutions, to no avail (seems most sites have the same (AI-generated?) info saying "Check your cables", "Contact your ISP"... basically the same thing the screenshots above say). One said to disable "Wi-Fi virtual adapters" that are known to cause network problems with Win 10, but no such adapter shows on his laptop, it just has the one hardware Wi-Fi adapter that is built into it (my own system is an older HP desktop connected via ethernet cable).


If it matters, the router is a TP-Link AX-1800 (a.k.a. Archer AX20), and Netgear CM-400 modem, however exact same issue happens with other routers (Asus, Airlink) and has happened with past modems too (whatever modem it was we previously rented from Spectrum), but figured maybe it needs mentioned anyway... A firmware update on the tp-link made no difference, modem already shows latest firmware.

Anyone have ideas they can throw at me as to what some causes might be?

TIA

 

I would gather timestamps of when you lose connectivity, then in your router check the logs. Look under advanced there should be a logging section. it could be your internet link flapping (going up/down). If you see your internet link go down in the logs you can screenshot it and send it to your ISP's support.

 

See, I'm not quite yet convinced this is something on the ISP side of it, only because the problem always goes away when my dad is not here... (like it has something to do with his laptop (or phone) being within Wi-Fi range of the router). But certainly that's a good point about keeping a log of events (which I'll start doing). Appreciate the suggestion.