In message , Harry
Bloomfield writes
Two possibilities, or maybe a combination of both...

1. The startup current of the motors is pulling the mains supply down
to the point where the router is no longer able to function and resets.

2. The interference from the surge of current is getting into the
telephone line or router via the LAN wiring causing it to have to
resync itself.

Alternatively, if it's a speed controlled motor then the interference
may well be from the controller. I think the OP has answered his own
question though, he says the cables run close to each other in the loft
so re-wiring the phones and running Ethernet to his PC from the router
at the master socket instead of the ADSL signal to the router is
probably the best solution.

For 1. you could try a small UPS or even a small 12v battery if the
router is able to work on that voltage.

For 2. make sure telephone and LAN cable are well spaced away from
mains wiring and perhaps add some ferrite rings around these cables.


--
Clint Sharp