Guest networks slow on upload on 5 Ghz, but regular networks are fine on 5 Ghz and 2. Ghz

Hello, Recently we have found that upoload speed on 5Ghz (AC) is just .1 or .2 Mbps (almost zero and shoud be approx 38 Mbps) but download speed is correct (C108-115Mbps). When signed in to same routers but not the guest SSID, the connection download speed is perfect.i-phones &  Samsung android Note20 phone indicates internet might not be present when attaching to 2.4 Ghz SSID as Guest.  When logged into the non-guest networks on these routers the connections (upload & download speeds) are perfect.  Wired ethernet to the Telstra Business Smart Modem is also perfect. 

The isue is the guest network upload speeds.  It is a new problem.  It has been working fine until the other day.  Speeds were tested with speedtest.telstra.com  It is not a browser issue.  Happens on mupliple devices: i-phones, i-pads, Samsung Note 20 phones and Mac laptop. All devics are within line of sight. Replcated the isue on a WNDR4500.    Have rebooted all devices (phones, Netgear Telstra Business Router, NBN Modem Box, WNDR4500) without improvement.

I could not find any setting that mght throttle guest connections on a frequency/channel. 

Can you please advise best course of action to get Wifi guest upload speeds working again please?  It must be somethign to do Wifi  as wired ethernet is fine.

Thanks,

Jeff