ASUS RT-AC86U and Spark Fibre

I bought a ASUS RT-AC86U last year and was getting random Internet drops every few days or so. I tried upgrading firmware from ASUS and Merlin to no avail. I took the router back and they replaced it. I configured it, upgraded to the latest ASUS firmware and was happy for a few days and bang same thing. Random Internet drop.

Since then I've used the standard Spark router and it's never dropped at all.

I like the ASUS router and it has great wireless coverage. But the drop outs are annoying

Any ideas would be great. I've seen articles referring to DNS, DHCP etc but nothing concrete

Thanks in advance