does an iPhone 7plus support 5G

No.

The oldest model that supports 5G is the iPhone 12. Nothing prior to that supports 5G.

Note that some carriers, notably AT&T will display "5Ge" in the status bar. That is NOT 5G. It is their "enhanced LTE" that they are calling "5G evolution"... It's actually 4G.

If you are referring to 5G cellular coverage, no.

5G coverage requires an iPhone 12 or 13 series.