An alternative to a different router is get a separate Wireless Access Point (WAP), connect that to the router via Ethernet cable, and disable the Wi-Fi in the router itself.
You can then locate the WAP in a suitable location which gets good reception around the house, which frequently isn't where the router is located which typically is near to the phone socket (and should be to minimise the cable run to the socket), often in the corner of a house. Your own WAP detaches the Wi-Fi from the provider also and you can switch providers without affecting that part and don't have to update devices if you change router. You can also get multiple WAPs to extend range.
TP-Link do some good kit.
p.s. Is the phone socket a single socket with just one phone jack, and if so are there extensions off it, or filters? If there's no phone connected, is it just one wire going from the socket straight to the router?
Test really is Ethernet connection from a laptop or similar direct to the router and do speed tests from that without the Wi-Fi.
