Have a look outside for a brown or grey box. Trace any cable from there if found. You may not have an internal box just a cable hiding somewhere. Property will have had cable at some time for you to have been sent self install.
Inside the house theres a bt openreach mk4 master socket which was fitted since we've lived here. Next to that is a weird old looking thing with a cable screwed in to it but ive never touched that, not even sure if it goes to anything?!