Powerline adapters are not particularly good, better options are available, and not necessarily a second line , before you order a second line , a word of caution, especially if using a different provider to whoever supplies the existing line ,( presumably that’s BT ) , although it’s possible to order an additional line , often the new provider issues the wrong type of order to Openreach, resulting in a migration of the existing line to the new provider ….before you go down this additional line route , is it possible to hard wire a ‘proper’ Ethernet extension to where you want it ?, most electrical/network contractors , electricians etc should be able to provide a ‘data’ extension or possibly run a long length , pre-made , Ethernet cable from the existing router to the required location .
Finally , your 25Mb seems a little in the low side , the upper end being 45Mb , your internal wiring/sockets or a line fault could be the reason your line is at the bottom end of the predicted speed , if you are a BT customer , post your router statistics they may indicate a problem, if a problem is indicated, you would have to prove it wasn’t something within your home , like extension wiring or sockets affecting the existing broadband service
