Orbi RBK53S having blocking outgoing connections from Synology NAS

Guys I need some help, I've recently been suffering with a problem whereby my Synology NAS (DS918+) is unable to access the internet (to update DDNS data, etc)

This has worked fine for 9 months but since the last Orbi firmware update (I think) this has been problematical. To debug the issue all network cables have been replaced and the NAS connected to various ports. If I switch back to using the British Telecom Home Hub 5 as a modem / router everything works.

Tests that synology have asked me to run give results as follows:

root@mynas:~# ping 192.168.1.254
PING 192.168.1.254 (192.168.1.254) 56(84) bytes of data.
64 bytes from 192.168.1.254: icmp_seq=1 ttl=64 time=0.357 ms
64 bytes from 192.168.1.254: icmp_seq=2 ttl=64 time=0.327 ms
64 bytes from 192.168.1.254: icmp_seq=3 ttl=64 time=0.423 ms
64 bytes from 192.168.1.254: icmp_seq=4 ttl=64 time=0.323 ms
64 bytes from 192.168.1.254: icmp_seq=5 ttl=64 time=0.233 ms
^C
--- 192.168.1.254 ping statistics ---
5 packets transmitted, 5 received, 0% packet loss, time 1002ms
rtt min/avg/max/mdev = 0.233/0.332/0.423/0.064 ms
root@mynas:~# ping 8.8.8.8
PING 8.8.8.8 (8.8.8.8) 56(84) bytes of data.
From 192.168.1.254 icmp_seq=1 Destination Port Unreachable
From 192.168.1.254 icmp_seq=2 Destination Port Unreachable
From 192.168.1.254 icmp_seq=3 Destination Port Unreachable
From 192.168.1.254 icmp_seq=4 Destination Port Unreachable
From 192.168.1.254 icmp_seq=5 Destination Port Unreachable
From 192.168.1.254 icmp_seq=6 Destination Port Unreachable
From 192.168.1.254 icmp_seq=7 Destination Port Unreachable
^C
--- 8.8.8.8 ping statistics ---
7 packets transmitted, 0 received, +7 errors, 100% packet loss, time 8ms

root@mynas:~# ping www.google.com
PING www.google.com (142.250.187.196) 56(84) bytes of data.
From 192.168.1.254 (192.168.1.254) icmp_seq=1 Destination Port Unreachable
From 192.168.1.254 (192.168.1.254) icmp_seq=2 Destination Port Unreachable
From 192.168.1.254 (192.168.1.254) icmp_seq=3 Destination Port Unreachable
From 192.168.1.254 (192.168.1.254) icmp_seq=4 Destination Port Unreachable
From 192.168.1.254 (192.168.1.254) icmp_seq=5 Destination Port Unreachable
^C
--- www.google.com ping statistics ---
5 packets transmitted, 0 received, +5 errors, 100% packet loss, time 7ms

root@mynas:~# nslookup www.google.com
Server:		81.139.57.100
Address:	81.139.57.100#53

Non-authoritative answer:
Name:	www.google.com
Address: 142.250.200.36
Name:	www.google.com
Address: 2a00:1450:4009:81f::2004

root@mynas:~# nslookup www.google.com 8.8.8.8
Server:		8.8.8.8
Address:	8.8.8.8#53

Non-authoritative answer:
Name:	www.google.com
Address: 142.250.200.36
Name:	www.google.com
Address: 2a00:1450:4009:820::2004

root@mynas:~# traceroute 8.8.8.8
traceroute to 8.8.8.8 (8.8.8.8), 30 hops max, 60 byte packets
 1  192.168.1.254 (192.168.1.254)  0.352 ms  0.392 ms  0.403 ms
 2  * * *
 3  * * *
 4  * * *
 5  * * *
 6  * * *
 7  * * *
 8  * * *
 9  * * *
10  * * *
11  * * *
12  * * *
13  * * *
14  * * *
15  * * *
16  * * *
17  * * *
18  * * *
19  * * *
20  * * *
21  * * *
22  * * *
23  * * *
24  * * *
25  * * *
26  * * *
27  * * *
28  * * *
29  * * *
30  * * *
root@mynas:~# 

As you can see outgoing connections to Google (used as a test) fail at the Orbi (192.168.1.254)

This is causing major concerns as I host a forum on the NAS and the issue takes it offline.

Any ideas how to fix it folks? Any assistance would be greatly appreciated. If any more information is needed please advise.

Thanks in advance.