It doesn't matter if it's apps that use data or apps that don't use data. If your employer requires you to install an app on your personal phone, you can refuse. It is your legal right. If you choose to exercise your legal rights, your employer must provide you with an alternative method that doesn't involve your personal phone. Whatever they choose.

If you agree to installing a work related app on your personal phone, you must be compensated. If they refuse to compensate, you're back to square one. They must provide you alternatives.

If your employer refuses to supply you with the tools to complete your job and/or refuse to compensate personal phone use for work related reasons, they are breaking the law. If they fire you for exercising your rights, it's unlawful termination.

Here's an example: My employer started requiring 2FA for the computer logins. They wanted me to install an app by Cisco. I said no. You can provide a locked down phone that can be used for the sole purpose of 2FA. They declined as that isn't in their budget and "unnecessary". They later came back with a little keychain that's bound to my account. I press a button on the keychain and get the 2FA code. I can do my job and they did their job and gave me the tools to do so.