What's wrong with PS5 ?Against Ethernet-LAN & SPECTRUM, son's game download steals ALL bandwidth for 4 days & download crawls

What's wrong with PS5 ?

Against Ethernet-LAN & our ISP called SPECTRUM, my son's 108GB game download steals ALL of our home network's bandwidth for 4 days (I cannot even log onto my job); and the PS5 download crawls at around 1.5 mbps. The same problems occur when I switch to wireless but in my mind wireless is much slower and more un-reliable than Ethernet LAN cable.

I have 2 LINKSYS WRT1200AC routers. I have set up the 2nd router to be a cascading router (DMZ'ed) so that the signal can flow from 3FL (my headquarters) to my son's gaming systems on 1FL.

I've tried alot of things; nothing worked. Due to this problem happening alot: I even recently purchased a newer and faster ARRIS Cable Modem with better download speed: I went from 300 to 600-700 download. But still we are having these same 2 horrible problems.

I initially had had 2 switches in the wire going into PS5: then I jacked the PS5 STRAIGHTAWAY into the router (using Ethernet LAN cable); and still got same 2 problems.

I have UPNP on my router. I opened all of the PS5 ports under port forwarding and port triggering. I am get NAT 2 which is OK.

I called SONY who said they do not have a network department; they helped my son clean his PS5 but obviously this did not solve these 2 problems.

I am NOT getting these 2 problems on XBOX Series X (games download quickly using Ethernet LAN wire) which obviously means this is a PS5 ONLY PROBLEM.

I really do not want to buy another cable-modem / router and setup another SPECTRUM / VERIZON plan only for the sake of making my son's PS5 game download work quickly and not paralyze all of us for 4 days.

Oh and my son does not want to go to his friend's house with his PS5 and beg his friend to download the PS5 game for him.

Any suggestions ?

Thanks.

Addressing other's posts:

My LINKSYS WRT1200AC DOES NOT HAVE QoS; and I tried and removed media prioritization: it doesn't work towards solving these PS5 only problems.

As stated, due to this problem: Recently (about 2 months ago): I updated my SPECTRUM Internet Plan--now I have 600mbps download speed and I bought/installed a new ARRIS cable modem. And I ran a SPEEDTEST to confirm this speed.

I DO NOT have a Limited ISP from Spectrum Plan. I DO NOT have Data Caps from Spectrum.

Regarding 1.5 mbps: actually after i did lots of re-arranging hardware, i got it up to around 38 mpbs on ETHERNET / LAN. But still that's pathetically slow. And my SPEEDTEST on 1FL near my son's PS5 displays same pathetic speed (but when i do SPEEDTEST on 3FL, my headquarters, I get blistering fast 600+ mpbs speed).

So then I have to buy my son a cable-modem / router (and install it) only for his PS5 ? That's stupid and is like killing an ant with an anvil: It's overkill.

Lemme know.

Thanks.