Greensands
Standard domestic market wireless telephone units are DECT (Digitally Enhanced Cordless Technology) systems which is very different from WiFi.
You can buy range extenders for these but they may take a bit of tracking down. £30 ish for off brand systems when I got mine. A bit more for on brand units designed to pair with specific phone sets.
They work quite well.
The off brand ones I originally bought to use with Siemens Gigaset phones worked fine out to my brothers garden office, about 18 yards, but struggled to reach further than just inside inside my workshop about 30 yards away. Replaced one with a pukka Siemens unit which is fine giving me a signal right to the end of the workshop, call it 40 yards.
Extenders are on second floor of a brick house next to the wall facing the office and workshop which are proper wooden buildings with cavity walls. Open air range is claimed to be over 100 meters.
I'm sure my second off brand one is still around somewhere, I've no use for it so if you fancy giving it a go you are welcome to have it for the price of the postage. Assuming I can find the instructions.
PM me if you are interested.
As I recall matters initial set-up was a bit challenging. Once it was all up and running it was obvious what the instructions for both phone and range extender were banging on about.
But not before.
Doesn't help that some of the things you have to do are time limited and if you over-run your allotted few second it all goes seriously pear shape.
Clive