New router - still dead spots

Thanks so much. Talked with Frontier today and improved signal when they disabled wifi from modem. She also suggested moving to center room but also thought an additional plug in the wall extender upstairs might help get the signal to the back room. This is an turn of the century solid wood and lathe wall house with brick chimneys so not surprised dead spots. Do you think that would help and if so which specific device. I had previously bought a tp-link that did nothing then the $300 nighthawk.