@Jekyll99 

Hi,

The word is that for routers that have been custom-tailored for ISP's and which they directly hand to their customers, the ISP themself is responsible for providing the customer service.
(my personal guess is that this may be some sort of condition the ISP's accept in return for being offered these routers at very low prices)

If you can't get a user manual for your particular model, then your next best option might be to use a manual made for another router model that has similar features and uses the same webpage design. For example: https://www.tp-link.com/en/support/download/archer-vr2100v/v1/
Sure, there will be some inconsistencies (like a few options missing or extra options), but still better than nothing.