Have been trying to get wifi extenders for MONTHS - how can this be fixed asap please!

Hello - I recently moved into a new home where Xfinity was my only option. The home is long and narrow, which means the wifi signal at the back of the house is very weak. 

Our xfinity-provided modem/router (because for some reason, we could not use our own?) is located at the front of the house. I paid for an additional Pro WiFi extender (monthly charge), which needed to be close to the modem. However the back of the house STILL does not have strong connection. 

After calling many times and finally getting through to someone, they told me that because of this I was eligible to be given 2 internet extenders for free. First they told me to go pick them up at a store - however my local store (Geary Boulevard in San Francisco) told me that they could not give them to me, only sell them. Then I called back and got a live agent, and they said they would ship them to me. 3 weeks passed, no shipment. Then I connected with someone through the chat function, they apologized and said there was an error on there end, and that they would be delivered asap. 

It's now been 3 weeks since then, and no delivery. I am beyond frustrated and would like to get this issue resolved asap. How can I ensure that our free wifi extenders will actually be delivered? This has been such a terrible customer experience.