I have clients who use the leased gateway and we shut down everything we can on the gateway and let the MX do DHCP. If you used the leased gateway it is best to call and have them disable the wifi and put it in bridge mode. Every time I have called the freak out if you disable it yourself. I have an MX64 at my home office and I have Xfinity, I purchased an ARRIS cable modem and did the same thing at home. So short story long, yes just plug the modem into the WAN port. Hope this helps.
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