Note my technical knowledge is a little bit "dated’ (read as I"m old ) so this will not be a technical reply
So consider a single band 2.4Ghz unit. If you are doing repeater mode you are connecting your router to the source on 2.4Ghz and then creating a WLAN network also on 2.4Ghz. Effectively you have half the 2.4GHz band (300Mbps/2 =150 Mbps) doing repeater duty and half doing client/WLAN duty (150Mbps)
If you have a dual band unit you can do the repeater component on one band (and get the full bandwidth of 300Mbps) and do WLAN/client on the other band (again with full bandwidth - 433Mbps).
Whether you use 2.4 Ghz or 5 Ghz for the repeater function will depend on your source. If you’ve only got a 2.4Ghz source then you’re “stuck” with 5Ghz for the client/WLAN component.
2.4Ghz has longer reach and better “penetration” , 5 Ghz has faster speeds but not as long a reach.
Have a look at an article I wrote here and have a look at the speed tests I did. Note that this is based on the AR750S, the big brother of the Creta (more powerful processor, external antennae, GB ports, more expensive but the principle remains the same) Using one band to repeat and the other band for WLAN/Client substantially affected speeds
I’m currently housesitting in a outbuilding thats 25+ metres away from the main router. I have a OpenWRT box (not a Gl.iNet box, but an old TP-LInk for power and range) running in WISP mode. For some reason it’s decided to repeat on the 5Ghz band but by turning off the 5Ghz WLAN on the TP-Link and using the 2.4Ghz for my WLAN I’m getting around 35mbps. If I use a 5Ghz WLAN on the TP-Link and connect to that I get around 17Mbps.
Of course there’s lots of other variables that also come into play (source strength, distance to source, distance to client etc )
Hope this makes (some sort of ) sense and helps