I recently installed the TP-Link X55 system with three units.  I'm using ethernet as a backhaul to all units.  Coverage in my home is *outstanding*, using a wifi meter I have full bars in every square inch of my home.

Wit that said, we have had issues with wifi calling ever since installing the decos.  We live in an area with no cell coverage, so wifi calling is very important.  Most times these days our phones just show "SOS" in the service bar.  Sometimes this goes away with a phone wifi reset, but many times it does not fix the issue.  I'm coming from a TP-Link Archer A6, and we never had issues like we do now.  I did some searching and found the following information in the community:

https://www.tp-link.com/uk/support/faq/1259/

I disabled SIP ALG as it suggested, and it didn't seem to have any effect.  It also suggests reserving an IP address for the phones to enable port forwarding.  Is this really necessary?  My daughter has friends over very often, and it will be frustrating to have to set this up for every person that visits overnight.  I haven't tried it yet on our phones, but will do so in the near future.

I was hoping that maybe someone found another way around these issues.  Please let me know if anyone has any suggestions.  Btw--our phones are iPhone 12 and newer.