Does it support a 5G Wi-Fi, which I now have? Did work with 2.4G
Yes.
The current models that are dual-band are the SS+, SS+ HE, Express 4K/4K+, Ultra LT, Ultra, Streambar and Streambar Pro
For other models check your model number via Settings > System> About, and look on this Wikipedia chart:
https://en.m.wikipedia.org/wiki/Roku#Feature_comparison
Any models listing dual-band wifi support both 2.4 and 5 GHz wifi bands. If they don't say dual-band they are 2.4 GHz only.
For the models that do support 5GHz only non-DFS channels are supported - 32 to 48 and 149 or greater.