Smarty Coverage claimed vs Actual

The Coverage Claimed: "Yes - Excellent service Indoor and Outdoor"
My Actual Coverage : NO Outdoor BARS (ever) but not "No Service"
Indoors is similar, but dBm readings are worse

What I need:
One Time Passwords arriving before expiry (if at all) would be a good start,
OTP by SMS is like London buses - None for ages, then three all at once

What I'd like:
Incoming calls to my phone, not to Voicemail when I AM at home. 
I'm 87 - Normally, I AM at home

About Me: 
I worked in IT from age 20 to retirement at 67
I still offer a Fix your PC service to my local community, so I can cope with tech to a fair degree
This issue is really about variable radio signal quality rather than nice, easy, digital logic
I hope my issues will ring many bells for many other users

My Environment Summary:
I live in a Victorian brick semi-detached house in flat countryside SE of Maidstone.
My office has excellent wifi, but no chance of mobile signal. (Thick brick etc)

BBand
It's an FTTC service today (15 MBps down, + 0.5 MBps up), 1.6kms distant from the cabinet
MAJOR CHANGE SOON:   FTTP will be installed next week

Phone Settings
WiFi is Calling is always ON - of course.
They say "Wi-Fi calling allows smartphones to make and RECEIVE [??] calls."
That's not my experience for Receive  - and not on SMS either
WhatsApp gives a far more reliable service, but callers usually use the conventional mobile network to make their calls.

How you can help:
I'd really like to understand what affects my phone's visibility on the mobile network,
and what I can do to improve it.

Here are some SIM card status results for signal strength
My readings were taken Outside on a 1st floor balcony
Mobile network state "Disconnected"
Best dBm: -114dBm Worst -127dBm

I have an EE SIM on the way to get some EE signal strength readings
Looking at EE coverage map, I think EE will be worse than SMARTY
O2 and Vodafone's claims are nowhere near

Spilly