How hard it'll be depends on the provider. My provider's customer support explicitly told me that it is technically allowed but it is their policy to offer absolutely no help with that whatsoever for "liability reasons", and I'd be on my own if I got my router stuck in a misconfigured bridge mode where I can't access any settings anymore to fix things. They also told me to find my subscriber credentials on my own if I wanted to use a 3rd party modem.