More DDOS(?) Problems...........

[#1]

I continue to keep receiving security alert messages from my router, such as the ones attached.
Can anyone please give me a definitive reason as to what they are and why I am receiving them, and also how to fix it so that I don't - other than turning off the router 
Thanks in advance.

[#2]

The ones in this PDF are almost certainly false positives, the source IP addresses appear to be Akamai or Level 3 - both large content distribution networks.
These are very likely to be data that a computer requested and should have received, but your router decided to block it. What were you doing on the Internet at those times? Something that uses UDP, perhaps streaming or gaming?

[#3]

Thanks for your reply, ejs.
The router was switched on but all networked (basic home wired and wifi network) PC's were turned off.
The messages appeared in my inbox when I logged on to my email client (Outlook).
The router is set so as to send me all security alerts by email.
These messages only seem to appear when the router has not been in use for a while - like overnight.
And then perhaps nothing for a week or more.
Further thoughts would be appreciated.
Many thanks and regards.

[#4]

see http://community.plus.net/forum/index.php/topic,104252.0/topicseen.html for some other information that is indirectly related but may be helpful.

[#5]

I didn't spot earlier, that in this PDF, the source port in all the messages is 3478, which points to STUN.
It could be due to the servers for something like Skype continue sending some packets for a while after you've closed the program or switched off your computer.

[#6]

Thanks Anotherone - interesting posts from your suggested link.
And thanks ejs.
My broadband firewall at Plusnet has been found to be set on High. And I've rebooted the router but I'm still receiving similar Security Alerts from it!
And I've run Malwarebytes, Defender, and security essentials, but nothing found. And my Broadband usage is pretty well average.
Any further thoughts then please let me know? Otherwise I'll just continue in being vigilant every day or two.
Thanks all again.
Bestest.