Tmobile home wifi gateway affects phone service

If you are getting 270 MBs down that is pretty good. The 15 MBs upload is primarily handled on the 4G LTE signal. The iPhone 11 as I recall is only 4G LTE. The iPhone 13 Pro does have 5G capability for the data handling. Be sure both have Apple's updates. If you see battery drain issues there are some things you can do to improve that. I have the iPhone 12 Pro and an iPad Pro and I have updated to the most recent iOS version. Calls can forward in via the VoIP to my iPad and MacBook Pro and I have not seen issues with the calls but I have a Nokia gateway vs either of the newer options from T-Mobile.

If the phones are closer to the gateway of course the wireless signal will be better than if they are in another part of the house with walls or appliances in between. Metal and dense materials will have a significant impact on wireless transmission so just keep that in mind. 

If you can improve the RSRQ, radio signal receive quality on the primary signal that will probably help. Signal receive strength, RSRP is great to have but reduction of noise and improving quality will provide better performance. If the signal quality is degraded and the ratio of noise to signal is such that the noise is impacting the signal you will have packet damage and loss which results in retransmissions which will cause things to be slower. Voice traffic which is real time flow uses lots of smaller packets but they need to flow without interruption. If you run speed testing and see jitter is high that does not bode well for voice communication over the data path. Just because you can send the voice traffic over the gateway cellular links does not mean it will be better. T-Mobile will preference phone traffic on the cells more than the home broadband cellular gateways. If your iPhones receive good signal strength you might try using them and not sending the traffic over the gateway cellular links. I say links as the gateway does use 4G LTE and 5G NR channels and on the cells phones do get priority. Keep that in mind and test your phones as such. The phones on T-Mobile's network will also have IPv6 addresses so I am pretty sure the flow from the phones will be possibly better not sharing bandwidth with other devices on the home network. You can put your iPhone into Field Test Mode and see its cellular metrics pretty easily. Very helpful actually. Marcar *3001#12345#* and poke about and look at the cellular metrics on the phone and compare those to what you see on the gateway. You might find the phones use different cell sources. They might have better metrics. You don't know what you don't see. It might surprise you to see the phone has metrics just as good or better. One can hope that can be the case.