Question about TP-Link Deco M9 Plus Triband WLAN Mesh Set of 3 (AC2200)

Hello together I have my Deco M9 in combination with the UPC Sunrise Connect Box 2 (I think a UPC product). When I configure the Deco as a router, I constantly lose the internet connection and sometimes the connection to a Deco. In Access Point mode, the whole thing works perfectly. Can anyone help me? (I would prefer router mode)