AIUI Ofcom were unhappy with Three a couple of years back for limiting how a "phone" SIM could be used, and Three backed down. So Ofcom's policy is that you should be able to use a phone SIM in your router. Or for any other purpose like SatNavs, which I did for a while but ceased for other reasons.
Whether Vodafone adhere to the Ofcom view if pushed, and are simply not telling you that you can do what you like, I'm not sure, but see this article
Make sure you read right through
There may be a slight drawback with a contract phone SIM in a router in that any service notices are sent to that SIM, and of course the routers don't alert you to such messages.
To get round that I set up an online account for the phone SIM so could log in to see if there were any. The main one on my PAYG being of course the data usage. With the Three data SIM that comes with their HomeFi routers notifications come to my mobile to tell me.
Vodafone admin systems for customers to access things like that could be different.
My broadband basic info/help site - www.robertos.me.uk.
Domains, site and mail hosting - Tsohost
Connection - Three 4G, tbb tests normally 35-45Mpbs down, 65Mbps off-peak, 9-24 up.
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