TalkTalk ISP tries to protect their customers from scams by blocking all remote access software.
Splashtop reached out to TalkTalk to not be blocked by default, but there are no plans to unblock Splashtop.
To allow Splashtop connectivity via TalkTalk, you will need to disable the Homesafe scam protection.
(Or, in TalkTalk- My Account- Security- Homework Time, turn Homework Time ON and set times to Block gaming and social media sites)
--Thanks to Lauren and TalkTalk Support
In some cases, the router DNS may be configured to use TalkTalk DNS servers. It appears their DNS servers may also be blocking Splashtop name address resolution. If the you still see problems using Splashtop with TalkTalk after disabling the Homesafe, please check your router's DNS settings and try using Google DNS servers (8.8.8.8, 8.8.4.4) instead.
https://community.talktalk.co.uk/t5/Articles/Using-3rd-Party-DNS/ta-p/2205279
--Thanks to Billy