SPA3120 operating as a router

A friend has a Netgear DM602 modem. I used to use the same modem. It
can operate in router or modem mode. I found that I had to operate my
router without login and operate the modem in router mode in order to
get operation under PPoA as opposed to the US system of PPoE.
This did cause some problems.
I note that the Linksys router/ATA SPA3120 only caters for PPoE so I
am unsure as to how well this would operate with the modem concerned.
I'd be interested to know of the experiences of owners of the SPA3120
in regard to getting it working with a modem whilst using it also as a
router to link a computer from the ethernet connection.
To clarify: Modem to SPA3120 and SPA3120 to computer - as opposed to
any other set up.
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It's goodbye Norton - hello free AVG until they stop ripping us off.
Why should we be ripped off in the UK?