@Dashlate 

Have you tried playing with QoS settings? See Using QoS feature of Deco. 

Also, as a quick test, you could temporarily switch Deco mesh to Access Point mode and see if it makes the difference. If it does, and iPhone12 gets better speed, then there is a hope: something somewhere can be tuned in Deco router mode settings.

If, on the other hand, even in Deco Access Point mode there is no speed improvement for that iPhone, it means whatever you'll try to do in Deco configuration settings unlikely to have positive effect. Access Point mode is very much unrestricted pass through.

Finally, have you tried different speed test tools? I prefer fast.com web page, works everywhere in a browser, gives consistent results.

@Alexandre. 

Hi Alexandre,

Thanks for your help. The M5 doesn't have QOS settings by the look of it. Also it already is in access point mode. 

My son has just tried fast.com and he got 29mbps!  

Still getting 3 MBPS on his Speedtest app though!!