AV receivers can generally handle only one audio stream. The ones which can handle two (or more) will specifically advertise that they can send audio to multiple zones. It looks like your receiver is one of these. Search the manual for "zone" and read up on how to set it up and use it. Page 64 has instructions on how to use the remote to select an input for each zone.
The whole point of an AV receiver is to coordinate audio and video signals, so it may not be designed with the capability to deliberately cross up two of those signals. If the AV receiver can't do that natively (search the manual), this problem may be more easily solved with an external switch box. https/www.amazon.com/TNP-Switch-Switcher-Selector-Output/dp/B071ZDGVG2
Rig up two of your speakers to the switch output. Send zone 1 and zone 2 to the switch inputs. Leave the radio playing on zone 2.
■When you want to listen to the TV, set the switch to zone 1.
■When you want to listen to the radio, you just turn down the TV volume, and flip the switch to zone 2.
If your speakers need an amp, you can add one between the switch box and speakers.