Thanks.
Setting the router's channel to 11 (instead of auto) solved half of the problem. Now the AP and the router can coexist.
However, the modem does not want to play. As soon as the AP is turned on the router can not communicate with the modem and of course, the connection to internet is lost, both wireless and cabled.
Any suggestions there?
(also have a question on if it's possible to set the device that is wirelessly connected to the network to choose the AP or router depending on which has the strongest signal; now I have to manually change on for example my phone when I'm in the basement or in the living room, but if I can't get the AP to work in the network it's quite irrelevant)
//m