The ability to select which interface to use, when multiple exist, in a host such as Synology which as 4, plus those added by docker, is a long requested feature.
Engineering has not yet provided that feature despite multiple requests
I have been chasing resolution of this for some 18 months (minimum).
I’ve taken it on myself to write the code for it. It’s about 2/3 done. When I finish it, I’ll give the whole package to Engineering. It should be close enough to exact for a ‘drop in’ fit.
Open up network settings from the control panel on the Synology NAS and go to network interfaces, click the down arrow on the right side of each interface and find the one that has your local IP address of 192.168.x.x and click on edit. Make sure there is a check mark in the “Set as default gateway” after you make this change you may have to reboot all your device such as the troubled TV. It sounds to me like the interface that list the IP address of 172.16.x.x is set as the default gateway.