if the connection seen on Technical Logs "information tab" is 70Mb/ etc
(Advanced section - left hand side middle) the you are connected correctly still.
(in the center of the information list)
Data rate: 1.14 Mbps / 5.93 Mbps
Then information about how it/the line has connected...
eg this is for an ADSL line
Maximum data rate: 1140 / 6348
Noise margin: 5.7 dB / 5.7 dB
Line attenuation: 25.9 dB / 47.5 dB
Signal attenuation: 25.9 dB / 47.5 dB
PLUS as in your case The LAN Speed recovers - which says there is no Line issue.
FYI: If you look at the technical logs (other tab) and filter for (Select) WAN
it will show the times it has connected and the data rate it connected at
dsl:ADSL link Up: Down Rate=6520Kbps, Up Rate=1140Kbps, SNR Margin Down=6.20dB, SNR Margin Up=6.30dB
2 possible thoughts come to mind
a) You have wifi interference
- the BT 'auto channel' selector is not doing a good job - so go pick a different one (1 3 6 or 11) than it picked for itself on 2G. Same for 5G.
2G is often congested and goes further.
5G is less impacted usually (has more bandwidth anyway, and goes less far)
b) BT enable BT wifi automatically and could be being shared..
The LAN performance however shows this is not possible.
eg should someone logon on to your BT wifi signal, they share your bandwidth. As I understand it, there is (today) no bandwidth limitation on that for the FON/BT-Wifi users. [ in the early days they have a limit as I recall ]
But if that (b) was the case, then the LAN performance would suffer also. - so I suspect Wifi interference (2G) as being the cause and a poor Auto Channel choice.
So go choose a better one
and see if that improves life.