Why would I not be honest Alfredino?
And I have a funny feeling You got me totally wrong…
Firstly, You took it a tad personally, secondly You reacted like if I had said the bank was right and You were wrong… it was not what I had meant.
Let’s clarify - I do not deny your reasoning about security or justify your bank’s lame excuses!
What I am trying to say is a simple message coming from my experience and the reality around us, which is: You are not going to win that battle against your bank. I am trying to save your time and to help You avoid more and more frustration fighting that lost battle. And I applaud Revolutionaries and people who want to change the system as they believe in what they do.
Banks do what they do because they calculate costs versus gain achieved and whatever ‘power ride’ affected bank adviser told You about security was one of lame, official excuses.
They also buy their banking systems from companies that calculate costs and do not support Custom ROM systems in their approach. I am not saying that is right! Again, I am trying to bring to You the reality of this world and how it works Alfredino.
Look, I asked Proton AG in Switzerland about Proton Mail and Proton VPN - if they support Custom ROM mobile devices and the answer was ‘no’. So, please think about it - if a decent, private in their approach Proton who do not need to know your name or another email to provide service to you, and provide full encryption between proton users giving you an option to send encrypted message to anyone where you give the recipient a password to read it on proton platform will not support Custom ROM systems as not enough users around for them to bother what on Earth can you expect from your state controlled bank?!
Have fun with FSFE when chasing your case through them, but in that organisations there will be a bunch of pillocks like in the bank’s governing body Mate! They are in cahoot. You will get nowhere through official channels… Don’t make it your hobby becoming a serial complainer as that makes your life miserable, forget people around you
I am joking here but I wish You well and please believe me it is not worth your time - you could be doing nicer things instead of fighting the useless system and maybe enjoy your life avoiding a lot of frustration as they won’t change unless there are millions of users with Custom ROM phones.
And do not tell me off that I am one of those who stand at the side and help the system not fighting it. If You step aside and look at the picture: you can’t fight the system with the tools it gives you - like going to FSFE who don’t give a damn my Friend. I am getting frustrated as well pi**ed off with plenty of incompetence, laziness, control freaks etc. around me, and I would love to ignore all of it and save my nerves plus keep peace of mind.
The sooner you learn it that you are in fact on your own against that big system and try to live a happy life aside of it the better for you. Do what you can for yourself: get de-googled phones, use for example protonmail, do not open gmail account even for a second. I wish one could get paid in cash and not use electronic payments as that is a huge controlling network tax including but we are held captive in this system and it will not change soon I am afraid.
So, if You can get my point Alfredino and not planning to get at the helm of your country or of the world to change the rules for us and how it unfortunately works, You really might do yourself a big favour, leave it where it is, trying rather to make your phone work with your banking app and not the other way round. And it is at the end of the day your personal choice if You want to waste your precious time to fight the bank as its Customer, go to FSFE, give them bad reviews, smear them here and there - to be honest they react better to bad reviews than to direct complaints by the way. Most organisations in U.K. ignore you but once you put a bad review on local trust pilot they immediately contact you! Still, they won’t start supporting rooted phones 
I wish You the best Alfredino, let me know what FSFE said unless You drop the idea of wasting your time. Cheers!