I've a Cisco switch which has no internal real time clock.
In Conf t I configured several public NTP servers

ntp server ..................................Sky ER115 Q box
ntp server 2001:630:306:10::C9
ntp server 2A02:C7F:C98:3E00::1
ntp server 0.uk.pool.ntp.org
ntp server pool.ntp.org
ntp server 0.pool.ntp.org
ntp server 1.pool.ntp.org
ntp server 2.pool.ntp.org
ntp server 3.pool.ntp.org
ntp server ntp1.npl.co.uk
ntp server ntppub.le.ac.uk
ntp server ntp1.isp.sky.com

It should
On my switch I get the following output

switch#show ntp status

Clock is unsynchronized, stratum 16, no reference clock
nominal freq is 119.2092 Hz, actual freq is 119.2094 Hz, precision is 2**17
reference time is 00000000.00000000 (00:00:00.000 UTC Mon Jan 1 1900)
clock offset is 0.0000 msec, root delay is 0.00 msec
root dispersion is 5119.57 msec, peer dispersion is 0.00 msec
loopfilter state is 'FSET' (Drift set from file), drift is -0.000001634 s/s
system poll interval is 64, never updated.

switch#show ntp association

address ref clock st when poll reach delay offset disp
~ .INIT. 16 - 1024 0 0.000 0.000 15937.
.INIT. 16 - 1024 0 0.000 0.000 15937.
.INIT. 16 - 1024 0 0.000 0.000 15937.
* sys.peer, # selected, + candidate, - outlyer, x falseticker, ~ configured

I'm not sure on configuring associations and I wouldn't have thought one would need to configure an association (key) with a residential gateway NTP source. I was therefore wondering if NTP is passed through the ER115 firewall.
I configured inbound allow on port 123 UDP on ipv4 and ipv6 from the appropriate public servers to specific IP addresses on my LAN I want to Rx NTP on.
(I assume the ER115 will open the random ports as necessary if requests can go out of 123?)

I'm getting NTP on my pi server, on my desktop and on my modded chromebook running a gnu/linux distro.. I think Windows 10 Pro is full NTP these days, only earlier releases used SNTP.

I just want to check it's not the firewall before going on the Cisco forums as that's the first question they ask to NTP issue questions.
My clock has been set to near the correct time (I set it about 1m30s out) and the timezone and DST to

(config)#clock summer-time BST recurring last Sun Mar 1:00 last Sun Oct 2:00

I opened the ports on the switch for a period to test:
access-list 101 permit udp any any eq ntp
access-list 101 permit udp any eq ntp any