May simply be time for a new router, these devices typically are powered on 24/7. Though some models will last for years w/out giving hiccups, others will be flakey out of the box, or shortly thereafter. I do recall you upgrading the firmware, so you should be good there. If not, you'd know it, the device would have been bricked, unless there were a safety mechanism in place to roll back to what was there.
So how does the Internet run computer to modem only, or have you tried already? This would go a long way in determining what's going on, if all is good for a day or two, then no doubt it's the router or settings. Or possibly even the Ethernet patch cable to the router, oftentimes the lowest cost cable is included. Cat6 would likely be best for your speed package, even at my much lower tier, there was a jump in download speeds by changing all Ethernet cables from Cat5e to Cat6, by over 5Mb/sec. Plus the short patch cable from the modem to router is Cat7. Upload was unchanged. So in essence, I jumped into the next tier by investing in new cables of the next standard.
Got all 5 of these cables for Newegg for less than $20, including shipping. There is likely a similar online site in your region as this one.
Being that the tech found one bad section of the cable, there could be another. This is an issue when splicing these cables, less is preferable. There has to be as straight of a path as possible from the street to your modem. Depending on the dwelling, such as an apartment complex, there may be one service panel, normally locked to prevent theft of services & from there a cable to each dwelling or unit. When there are multiple services by the same provider, there will often by two or more to each, usually no more than two. Unless in the dwelling, there's those who wants separate accounts, then another can be added.
It's from that service panel or box, that any additional splices, T fittings & splitters must be kept to a minimum & when added, properly assembled. Some of these techs takes pride in their work, others are in a hurry. Back when I was a homeowner, there was a problem with one connection & fixed it myself by purchasing the fitting at a local Radio Shack & redoing it myself with a premium one. The difference in price for a pack of two was minimal.
To sum it up, the cables or lines all has to be as perfect as possible, otherwise no matter the price or quality of the modem & router, it's all for nil. You'll still have troubles.
Hopefully some time in the next day or so, someone can get this thing right. You & your housemate has the right to receive the services that's paid for.