I just scanned my port 2420 and indeed it is open. I also have Remote Access not enabled and the URL on the modem configuration page links to port 2420.
I am guessing that this port is alway open, but will immediately drop connection if Remote Access is not enabled.
Unless Verizon has a back door here
.
Or this port needs special protocol that script kiddies can run
Anyway, preliminary search on this site, there are some discussions on this port. One example is:
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There is a remote desktop function specifically built in for helpdesk access on port 2420. However it is a only on a per connection basis and it has to be enabled on the customer side.
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