Can u play games on 5G network on I phone xr

The reason that you can't "play games on [a] 5G network" on your iPhone XR is that your iPhone XR does not have hardware to connect to 5G cellular networks. It came out two years before the first iPhone 12 models and does not support 5G cellular networks in any way.

Use 5G with your iPhone - Apple Support

"iPhone 12 models or later work with the 5G cellular networks of certain carriers."

Many carriers have shut down their 2G and 3G networks, so my guess is that the only way you're getting any cellular service at all is using 4G/LTE.

As for why you can't play Roblox on 4G/LTE, I don't know. I don't play Roblox at all. This may be a question for your parents (have they put parental restrictions on your phone?), Roblox Support, and/or your carrier.

Roblox Support – Roblox Mobile System Requirements

"Roblox requires a minimum of a 4-8 Mb/s internet connection. If you experience problems and are currently connected to your phone company's 3G or 4G network, please try connecting through WiFi instead for a faster and more stable connection."