But have you cycled the router and modem ?
This has been an issue I have been struggling with over the past few weeks after moving into my new apartment and after speaking with IT, ISP and various technologically savvy friends who all do not know what is going on, I have now turned here for help.
The Problem: The internet cuts off completely happening at intermittent times. This is confirmed with the internet light on the router going out while all other lights remaining on (power, 2.4/5 GHz, and LAN).
Timing/Frequency: I first noticed the internet drops when transferring large amounts of data, like when streaming a movie or a radio, or downloading a large file. It also seems to cut out when another device joins the network, though only sometimes. During Speedtest, the internet stays strong through the download test. However, when it starts the upload test, it initially starts out strong then quickly drops to zero and the internet light goes out on the router again.
Hardware: Asus RT-ACRH13 - Firmware updated - had this issue before and after firmware upgrade.
My Thoughts (which may be wrong): (1) With the internet dropping with large data transfers, it may be a problem with ISP -> should contact ISP. (2) Internet also cuts when another device joins network -> possibly overcrowding network, should decrease number of devices connected. (3) Internet drops during upload test -> could be ISP regulation, could be issue with router or its settings.
What I Have Done: (0) Simple troubleshooting (because duh). (1) I have contacted IT/ISP and they show no issues on their end, which ruled out ISP momentarily. I also connected my computer directly to the network instead of through the router and the internet works just fine. No drops, no slow-downs, large data transfer and streaming fine, no issues during speed testing. This completely ruled out ISP for me and think it must be issue with the device. Anything connected to the router through LAN when the internet drops also loses internet. (2) Issues occur when only one device is connected to the network, so it can't be oversaturation of the network. (3) This is the doozy for me, I'm not sure what it means when the internet stays strong during download test but cuts out during upload test consistently. I haven't configured any settings on the router as I don't know what would help with this presentation of symptoms.
I have talked to IT, who do not know what is going on. Though, I have yet to speak to the manufacturer of the device (which I likely should have done before posting this, though I would like to know if this is a fixable issue first). I have tried searching the internet, though I have found nothing that matches the presentation of symptoms that my network is having (or maybe I'm just not that good at search queries). Asking for help here is my substitution for an unsuccessful internet search and will contact the manufacturer if the issue cannot be resolved here. Is there anyone on this forum that could help me? Thanks in advance.

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic