I think you'll find this post most interseting...
https://www.pcreview.co.uk/forums/thread-2061977.php
It's a way to be logged in as administrator, but not suffer a lot of the security holes we face when doing so.
As for the icon not appearing, Guest accounts are very *very* restricted as to what they can do. I'd suggest giving the guest account 'User' level access (by adding it to the users group, via your admin account).
If that doesn't fix it, I'd be interested in knowing the steps involved for you to connect to the internet.
If the guest can go to dial-up networking and see the modem connection, they should be able to double-click it to either dial, or bring up the 'currently connected' window that would pop up if you double-clicked the icon in the task bar. (if guest cannot see the modem connection in dial up networking, then we know this is a security based issue).
If the guest account is the one executing the connection, then they should be able to see the 'connected' icon in the task tray. But, If it's invoked by the admin account automatically, then it's running on that account, hence, guest wouldn't "see" the program (or the connection icon).
Imagine being connected to a network, where your PC isn't the dialer. If another PC is your 'host' and dials the connection, the 'host' would see the modem connection icon, but the client would not.
This really seems like a permissions based issue, in that the guest account has no access to view/modify/change connection details.
Questions:
Has this ever worked before?
If so, what has changed? (i.e. fresh install, or new account creation).
Who created the actual modem connection?
Is it asked to force dial upon detection of an internet request? (if so, I'd disable this).
What security "groups" is your guest account assigned?