I run a Synology NAS over a wired network to the Vero and can play 4K mkv files just fine (have never tried 4K m2ts though). Couple of recommendations for you as you say you are connecting via SMB.
Step 1: go to your Synology Control Panel, File Services and then enable NFS, then configure NFS permissions for your video share. You’ll need to create an NFS rule for the share, confirm IP addresses permitted to access (or you can allow all devices with “*”), set read/write privilege and “Squash” (I find with OSMC this has to be either “Map all users to admin” or “Map root to admin”).
Step 2: add a new media path in Kodi for NFS and check that you have playback. I found playback was fine but I had one title that gave me a chapter-skip issue where the buffer could not recover before playback stuttered.
Step 3: you now know that NFS is working. Potential further steps: either re-mount the NFS share via fstab (which has much better buffer recovery after chapter skip) or get yourself a very cheap USB/ethernet adapter. Happy to post a link to the one I bought. I went for the latter option for reasons I won’t dwell on here, but fstab+NFS certainly works very well in many use cases.