Sunrise has the Fritzbox routers 7390 and 7490 on loan. The Fritzbox 7490 supports FritzOS 7 and thus Mesh. The Fritzbox 7390 no longer works with FritzOS 7 and therefore no Mesh. In principle, WLAN repeaters can be linked to any WLAN, and the 1750E can also be integrated into the network with an Ethernet cable. The configuration is less convenient and can degenerate into a tedious process. Therefore, my recommendation: At Sunrise, insist on the delivery of a Fritzbox 7490 and, in an emergency, exchange it again. This will surely solve your problem.