Sky ADSL odd effects :: Sky Broadband

Anyone on Sky seen these effects? A friend of mine has the old Sagem router 2504n of which the WiFi is turned off and a dual band TPLink router is used as a WiFi access point. He has a 22mbps sync on ADSL so doesn't see the need to upgrade to FTTC.

* Facebook app on iOS (various iPhones and iPads) doesn't show pictures reliably
* Apple macOS Mail app cannot get mail from Gmail via POP or IMAP - just hangs forever

Using a VPN to my own router on Plusnet solves all the problems; so I've assumed its an ISP issue, as everything else (Netflix streaming, Sky OnDemand etc) works fine.

Anyone else seen these, and is this a router issue, or a Sky ISP issue?

plusnet unlimited fibre 80/20 since 2 Jun 14 - Sync as of 9th Apr 17: 56,605/9,592 kbps with G.INP
_{18 years of UK broadband since 1999 ntl:cable modem trial - Asus RT-AC68U and HG612 - BQM}