Hi! This is my first time posting so I apologies in advance if I'm using the forum wrong.
My current situation has been going for months as I tried to receive help from my ISP but this is also the first time they encounter this issue so I am coming to this forum site for help.
I found this issue as I only moved into this house months ago and the previous owners were an old couple so I figured they didn't noticed any issues with their internet usage. extra info: the ONT and modem is located at the kitchen
There may be some things I may have missed out. if I remember the extra information I may have left out, ill see myself update it.

Issue:
When I'm connected by either ethernet or WIFI. the internet seems to not connect to servers or services properly. most applications would tell me that its a networking issue such as my bank app or a game that requires a stable network connection, even google related apps and services have trouble loading for me. at this point its unbearable to use the home internet to where I resort to using my personal phone data to access or connect to certain stuff. (note im still connected on the wifi, its just the actual connection between the device/modem to the servers that seems to be the problem.)
I'm currently on the FibreMax plan with Spark (changed from the Fibre100 Plan as it was a potential solution. it was not) and I'm getting around 40 to 600 Mbps DL and around 1-300+ Max Mbps for Upload
-Connection is unstable. most apps, sites are telling me to check my connection or not load at all until a couple refreshes or app relaunches. game wise, its error: check your connection, connection to the server failed, error messages regarding to either networking error or firewall, or it connects but reconnects every few minutes.
-when connected to said site, server, service, etc. it feels fast thanks to the speed. but randomly the connection just feels like its either throttled hard or its fully cut.
-Examples:
-Steam, when downloading, its fairly good. but sometimes download seems to die out to 0 mbps randomly. minutes later it goes back up to its usual speed.
-Youtube (mobile app) also does this by either allowing 1080p videos or not loading the actual app at all.
-Facebook Messenger seems to do this a lot, the app sometimes doesn't send at all. if i send multiple texts at once, it takes a couple seconds to minutes to send and sends all messages in one batch at once when the app decides to connect.
-Twitter wont load anything but videos sometimes.
-Microsoft Teams will reconnect me multiple times during meetings randomly.
-Certain emails takes minutes if not half an hour to send a single email.
-Mobile Games wise, Pokemon Masters Ex is a great way to check if the internet still has the problem or not as it immediately tells me to check my connection. Genshin Impact either says network error or I'm in the game but i reconnect every second or minute.
(Examples above i have tested in other networks and shows no problem loading at all. tested both on pc and mobile.)
-Speedtest shows high speeds. but connection issue still persists despite high mbps.
-Speedtest sometimes show random dips during test such as going to like 100mbps then dipping randomly halfway to 0. not sure if its normal or has anything to do with the issue but that's there.
-TraceRT in CMD shows either steady pings every hop, the *** results, or it just the cmd just doesn't load the first hop.
-All devices, mobile phones and computers shows the same issue when on this network.

Things I've Tried:
-Anything Modem related like changing DNS gateway and changing the 2.4gz WIFI channels to 11 or 9 was troubleshooted by both spark's customer service representative and their technicians over the phone and TeamViewer. (vague i know. but if there's a certain thing you think might have not been touched in the ip admin panel from the technicians. let me know and ill doublecheck if I did troubleshoot it or not)
-Tested on the Huawei HG659b(previous modem before house change), 2 Spark's Smart Modems.
the The Huawei performed worse than the smart modems however all 3 still shows the same issues.
-ONT was changed from the Model type 100 or 200 to the Model type 300
-checked if it was any connected devices that was affecting the network and confirmed was not the cause
-changed the cat5e cord(from the smart modem box) connected from the ONT to the modem
-Spark has told me there's no issues from their system
-Chorus has told me there was no issues in their circuitry
-Changed from Fibre100 to FibreMax incase it helps change the profiling in the chorus network.
NOTE: troubleshooting has been down on both hardwired and wireless pc/laptops and my mobile phones.

the other thing I was recommended to try by the technician was to change the location of the modem, however this will take some time as I don't have a long ethernet cat5e or cat6 cable, only short and I recently have no time to troubleshoot as much as I was able to during lockdown. I was also told by the technicians a third party modem could potentially fix it however without a guarantee fix. I'm not willing to purchase one for the sake of troubleshooting due to cost reasons.
Another thing is that my previous house I used to live in wasn't as bad as this. the only issue was the network's mbps speed as the Huawei modem spark gave out was terrible. but it was never as bad what I described right now.

If there's certain information on the issue you want me to provide let me know.
its been a solid couple months and Spark and myself has hit a brick wall with trying to troubleshoot this issue. if any of you know or have encountered a similar problem to what I described, I will appreciate all the help and potential solutions.
Thank you.