2) buy an end-of-life Cisco IOS router (e.g. Cisco 1801 (ADSL over POTS), Cisco 1802 (ADSL over ISDN), Cisco 1841 + HWIC-1ADSL + HWIC-AP WLAN). They are now super cheap, but they aren't easy to set up, because they use Cisco's crown jewel IOS (Internetworking Operating System). On the flip side, there are plenty of sources to learn and get help. This is rock-solid industrial-grade solution for businesses and it was quite popular before VDSL or optical fiber pushed it out of the market.
How do I choose, which Cisco will work with my ADSL line?
POTS, ISDN, or Cisco 1841? I have standard UK telephone line, 21Mbps down 1+ Mbps up, all routers with R12 (that's how it's called, right?) socket are physically compatible, but I don't know which router to choose.