Download speed for 2degrees.

Hello. I'm wondering, as an internet user, how do I determine the slow internet speeds is MY PC fault,  the modem or the host (2 degrees).

I used to live on my own & had full control of buying/managing my own modem etc. I'm now living in a shared accommodation, & only use the internet as a user.

I do know I'm on 2degrees. however took a month for me to know they "THINK" its a fibre connection.. 

I'm using the internet at 100+ Mbps and then it goes to a crawl at 0.2 Mbps (zero point two, or zero point 9 etc..). If the internet speed is too slow, its not even recorded with Speedtest...

https://www.speedtest.net/results?sh=7276c9efb292932716152e14461d9bbb

How do i know 
a) its my PC, 
b) their modem, or 
c) something to do with the connection itself ?

its now 1:40 AM on Sunday, and I just got a 0.27 Mbps download speed, and then this error from Speedtest:

DOWNLOAD TEST ERROR
A socket error occurred during the download test. A firewall could be blocking the connection or the server might be having some issues. Please try again later.

Alternatively, check out our free Speedtest for Desktop apps for more reliable testing.

(Speedtest website)

I tried the Speedtest APP (windows 11) and I am and now 100MBps 

retried using the website and its also at 125Mbps + 

How do i fix this (if i can) as a user to get REGULAR high speeds at all times ?  socket ? 

its now 1:43 am and I *KNOW* everyone is asleep before 10 pm..  I'm the only one awake..  
PS I don't go on 'newsnets' or other sites, just regular sites, games, Google, Gmail etc.. 

Also, the only one that knows how to use / control the modem doesn't live here (local staff only turn modem off and on)

Can i fix this "socket"?

EDIT: i do have full control over my PC