Hello Tacheles
Your message is not entirely clear. I assume that you have Synology MR2200ac, Synology DS218 and a Fritzbox 5490. How should the devices be used, resp. which services, e.g. DHCP should they provide?
The DS218 is not a router and will therefore never be able to recognise other devices. MR2200ac and 5490 are both routers and you have to decide which one you use as router and which one you use as WLAN access point. As an access point, the MR2200ac must be connected by cable, as it cannot be used as a WLAN range extender (repeater). The 5490 can be used as a WLAN repeater, but according to the instructions only with other Fritzbox devices. Ergo, the 5490 would have to be connected by cable to the MR2200ac as a router.
However, it is clear that the MR2200ac and the 5490 cannot create a WLAN mesh. For a mesh, devices from the same provider would have to be used. This means that the two devices can each provide their own WLAN.
My recommendation is to use a provider for the WLAN that supports a mesh network, such as AVM Fritzbox or Synology. With the Synology, the RT2600ac would be recommended as a router.
HG PhL