Repeater, VPN, and chromecast. Error: "Network device not present"

Hello I am no expert and would very much appreciate your help on this matter.
My goal is two things: 1. make a repeater out of an old router with wireguard vpn on the router, and 2. create an access point on the router that is able to connect to chromecast.
Step 2 is what I am struggling with. Router name: Asus AC750. The router has two radios (radio0 and radio1). I have understood that the repeater(client) and the access point(master) should be created through the same radio. Creating a repeater and access point through radio0 works fine. However, the access point through radio0 cannot be recognized by chromecast (although other devices such as my phone can recognize the access point and use it). On the other hand, if an access point is created through radio1 the chromecast will recognize it. The problem here is when I create a repeater through radio1 and apply wireless security encryption for the access point on radio1 the router will no longer connect to internet. I get an error message under the interfaces WWAN and WG0 (wireguard) saying "Network device not present".

How can I make it work?

Thankful for all help!

Actually it is OK and often better to use the two radios separately for in and out. Which radio is 5 GHz? Is the Chromecast capable of 5 GHz?

Thanks for the reply. Radio0 is 5 Ghz and radio1 is 2.4 Ghz. Chromecast is gen1 and is only capable of 2.4 Ghz. When the radios is used seperately for in and out it works but the internet download speed is down to unusable levels. Upload speed is ok.

Note: In other words; speeds are slower than usual when radios is used seperately for in and out.