ISP still throttling my bandwidth despite successful installation of Tailscale

Windows 10x64 Home Edition with latest updates.
Tp-Link M7530 4G+ Router connected to a Ravpower File Hub in bridge mode.
(4G+ unlimited Internet plan with Yoigo Spain who proposed this service as a Broadband alternative since there is no Fibre Optic service here.)

True to their promise I found the installation very simple and straightforward on all my devices (iPhone and Ipad Pro and 2 ancient Windows laptops).
However being in a rural part of Spain unfortunately I can only connect to the Internet via my Tp-Link 4G router (or iPhone 4G hotspot), and since using this service I have noticed a significant disparity between my download speeds as registered on Speedtest and other similar apps which show quite reasonable download speeds of around 20Mbps, and the speed shown by the Internet Speed Test of Fast.com, which only registers around 2.5Mbps. This leads me to the conclusion that my ISP is throttling my bandwidth for activities such as streaming or torrent downloading etc. I have tried several different proxies and VPN services, most of which reduced my download speeds even more and also made it impossible to load several different web pages.

Forgive my ignorance as I confess to being a complete newbie, but what I would like to know is whether there is any way to configure the tailscale VPN such that it bypasses these restrictions, since even 720p streaming is impossible?
Would this necessitate my installing the VPN in the router itself, or on the Ravpower Filehub - would that make any difference?
I don’t understand why my ISP seems to be still able to see everything I am doing online despite using this VPN. I have configured two alternative DNS services as well, and I see that my IP address is different from that of my router.

BTW, I have verified that torrenting is now impossible whereas with my previous ISP there were no such restrictions, but this was a Wimax service which was too restrictive in other respects as well as being more expensive.

Thank you, any help or advice appreciated. David