It all depends on the technical requirements of your ISP (and your WAN speed!).
If you're on an ADSL/ ADSL2/ ADSL2+ contract, the modem isn't going to work (Annex A).
For a VDSL/ VDSL2/ VDSL2+vectoring connection, the modem should work - and rj-45 to TAE cables are easy to get (1-2 EUR, so shipping would be the biggest expense).
With the few fibre/ FTTH connections or the various hybrid contracts (supplementing ADSL with LTE), the modem won't be usable.
In general the Linksys WRT-1900ACS should be a decent router, capable of routing WAN speeds up to 1 GBit/s wire-speed, but it does not include an ADSL/ VDSL modem, nor can it provide VoIP/ SIP services (yes, it can obviously pass SIP traffic through to a dedicated SIP phone or SIP pbx, but you can't connect your existing analogue phones or DECT handsets to it). This means you may have to add additional devices for using the phone capabilities of your contract (if so desired).
There are a couple of modem-router devices (the lantiq VRX200/ "VR9" series) supported by OpenWrt (your Netgear DM200 is one of them), but their performance as router is significantly below that of the Linksys WRT-1900ACS (mvebu), even more so if you want to use FXS (phone) capabilities (via chan_lantiq and asterisk) as well. So your strategy of using a dedicated VDSL modem and a discrete router makes sense (especially beyond 50/10 MBit/s), supplemented by SIP capable devices as needed. The BT Home Hub 5 Type A would be one of the most popular examples for an OpenWrt compatible device combining a VDSL2+vectoring capable modem and concurrent dual-band wireless router into one device (it can do up to ~85 MBit/s routing without flow-offloading, around 100-115 MBit/s with flow-offloading enabled, meaning it's sufficient -but borderline- for a 100/40 MBit/s vectoring connection; devices with FXS ports will be limited to around 55-60 MBit/s, as they have to reserve one CPU core for running the voice firmware).