Zyxel VMG3925-B10B - Showing huge packet loss on TBB BQM???

Recently got my hands on a Zyxel VMG3925-B10B VDSL modem/router from eBay. Unfortunately it was locked to KCOM so I had to unbrand it first to get it working with my Plusnet connection. I used the ROM-D technique and used "save_default clean" followed by a firmware upgrade - following procedures I found on the Kitz forum.

I am using the VMG3925 as a modem in bridge mode with an Asus RT-AC68u acting as the router. When I set up my TBB BQM I notice huge red packet loss with clean intervals in the middle as shown in this graph: BQM

There are no options to allow ICMP in the GUI like there were in my previous VMG8924 that I had before selling soon after. Steps I followed on the VMG3925 to no avail:

- Disabled DHCP (its in modem mode anyway)
- Disabled NAT
- Disabled Firewall
- Disabled QoS
- Disabled WiFi
- Lan4 is connected to Asus router's WAN port so I set up a separate interface group for Lan4 under VDSL
- Lan1 is connected to Asus router's Lan 2 port so I can use DSLstats
- Disabled DoS protection
- Under Remote Management section there is a row called "PING" - I set up a trust domain with the IP block used by TBB's BQM and allowed it

If anyone could help me troubleshoot this issue would be greatly appreciated. AFAIK there is no discernable effect on actual throughput but the considerable packet loss makes the BQM meaningless for future diagnostic purposes. I was using a Huawei HG612 modem with my Asus RT-AC68u router previously with no issue.

Many thanks