Internet in detached garage office

As far as I know you can't have multiple accounts to the same residential address without jumping through hoops.  That's ok because it's not the best way to do it anyway.  Usually I would suggest running fiber to outbuildings like a garage since a cable flying through the air is a risk for lighting strikes, but you buried coax in conduit underground so that's not a concern.  Since you have that coax I'd suggest looking into MoCA adapters.  MoCA is basically ethernet over coaxial cable.  You plug an RJ45 ethernet cable into one end of the adapter and coax into the other.  You'll need two, one on each end of the coax run to the garage.  The latest versions can do 2.5Gb speeds just as well as cat6 on a point to point connection, though the equipment does cost a bit more.  Might cost you $150-200 for the gear, but that's so much cheaper than trenching... and that's for the 2.5Gb stuff.  1Gb MoCA gear should be cheaper.  One thing to keep in mind is that if you do set up MoCA on coax that's also connected to Comcast through splitter or whatever is that you need a MoCA point of entry filter between your home network and Comcast.  If you don't have one any MoCA capable gear your neighbors on the same Comcast cable segment have may end up attaching to your network.  MoCA PoE filters are like $10 on Amazon, so it's no big deal.  Just something to be aware of.

@user_0ed672 

Adding, that if you do decide to go the MoCA route, and there is / are a splitter(s) in the mix, it / they need(s) to be MoCA compliant (5-1675 MHz) such as this one; https://www.amazon.com/Amphenol-2-Way-Digital-Splitter-ABS312H/dp/B08CRQLG8T 

Hello @user_0ed672! Thank you for reaching out over our Community Forums. @zandor60657 and @EG have provided some great information on using MoCA adapters. However, if you were provided an additional Gateway, you may be in an area that does allow the use of two modems in the home. We would have to double check for you, please send us a direct message with your name, and service address. 

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And if you are in an area that does allow two modems, you'll have to pay for two accounts (twice as much as you are paying now for internet) forever.  Other options will pay for themselves in fairly short order. Only you can decide what is economically feasible for your financial situation.