Maybe this is part of the issue and you shoulda used proxies.
It's true that the ips will be different with each reset, but still belong to the same neighborhood, same ISP or same 3 ISPs, if the dongles belong to different providers.
Also how many accs were used with the same ip? You mentioned, that you had 300 accs total, you used 50 at a time and 3 mobile dongles with periodic ip resets. If you ran the 50 accs simultaneously, you used 16-17 accs on the same ip. Or did you do it like that you ran 3 accs at any given moment and when they finished their task, you reset the dongles and moved onto the next 3 accs?
Imo, and from my own success, proxies are not required. I've detailed this in another post but here's the theory ...
Mobile networks have millions of customers. But they don't have millions of IP addresses. The availability of their IP addresses is limited to what they own and have available. Thus, there isn't enough IP addresses to give every customer their own unique IP. That's why when you reset a dongle you get a fresh IP. The IPV4 shortage is real.
The majority of Facebook users access the site from mobile devices. If I visit Facebook on my phone now, and then visit again later, there is a chance I'll have a different IP address. Facebook knows this, which is why they can't base their algorithms on IP addresses alone.
Also, it's very possible that some users will share an IP address. Lets not forget that many ISP's also implement Carrier Grade NAT which means many homes will share an IP address within a small regional network. Facebook have to take all of this into account.
Anyone managing a lot of accounts will be using a dongle, or multiple dongles. Last week I was sitting on over 1000 before the sweep ban happened. I'm just less than 1000 now so I still have a lot of accounts available that are not affected. The ones that were are related to what I was doing and without those accounts a huge source of income is gone.
When accessing Facebook using cookies, instead of manually logging in, you also bypass a lot of device and location checks. That's something a lot of people don't know. I could send you the cookies to my accounts and you could access them without logging in and the accounts will remain alive. This is also what many PVA sellers do.
Take all of that into account and you can see why proxies are not required. It would get far to expensive.
Just my 2 cents on that

Previous to this sweep I've never lost accounts like this.