Sophos network agent : failed to connect

hello

we had XG 460 rev2 with sfos 17.5.15 , and all our android clients work perfect using Sophos network agent v.1.6.

but when we upgraded to SFOS 18.5.1 MR-1-Build326, all android mobiles still work using network agent

except mobiles with old android ( android 5 , 4.4.7 and below).  network agent always give error "failed to connect"

Any ideas

thanks