Fulfilling Written Statements

I am attempting to exchange an iPhone 18 Pro Max for an iPhone 18 Pro. The reception is poor compared to my spouses iPhone 18 Pro when we make calls from the exact same location.  We were originally told in writing from the Xfinity Store that we could complete an exchange, but now they are telling us we can't complete a second exchange. The return policy available on Xfinity's website doesn't state that only one exchange can be completed.  We also reached out to Xfinity through the online chat multiple times and they told us in writing that we could cancel the exchange or complete another exchange, but we had to go back to the store.  We did this twice. They assured us that they put the appropriate notes in the system and all we had to do was go into the store. I even asked them the second time that if we went into the store again and they weren't able to help us if Xfinity would provide us with substantial compensation and they replied back in writing that they would. We have reached out to Xfinity several times and have asked to be put in touch with "managers", but Xfinity won't follow through and deliver on what they promise. How can I get Xfinity to follow through on their written promises?