What's the best type of Ethernet cable to use with T-Mobile 5G Home Internet?

Most things should be fine.  RJ45, Cat5e, Cat6, or better.  

Internet Customer Question wrote:

Hi, question, I am using the T-Mobile 5G Gateway  SAG FAST5688W.  Do you think Cat 8 is better than Cat 6 Ethernet Cable?  Please advise. Many thanks!

It makes absolutely no difference because the port is not capable of reaching the rated speeds of either cable.

Thanks for taking the time to reply, syaoran. However, my question isn't which choice would be "fine" or adequate, but which would be optimal for speed. If you believe, based on technical expertise, that the type of Ethernet cable won't make an effective difference (because perhaps in this situation, the modem itself would be the most limiting factor on speed), then please make that clear.

Well, either of the cables I mentioned would be optimal.  If you anticipate good speeds though, then Cat6 is what I would start with.  The modem is limited by the Gigabit Ethernet ports as to how fast you can get, so even if the modem could pull 2 Gigabits, the ports are limited to 1 Gigabit, which in reality, is about 940Mbit as an average best case.  Cat6 or Cat6e is plenty good for those speeds and it's usually cheaper than RJ45 with longer cables. 

Cat 5e works fine with all my devices. IMHO, you do not need Cat 6 or better rating.

If you plan to connect more than one device using Ethernet, do get a good Gigabit switch. I am using a Dlink gigabit switch which works fine with my computers.

Hi, question, I am using the T-Mobile 5G Gateway  SAG FAST5688W.  Do you think Cat 8 is better than Cat 6 Ethernet Cable?  Please advise. Many thanks!

Bingo.  Biggest factor will be distance.  At best, you'll approach 1gbps on a gig port, due to overhead.  I actually get slightly better than a 1gbps over wifi in close range.