I am using GL-AR750 as a router and connected it to Synology NAS via ethernet cable. I cannot login to my NAS however. When I use ANY OTHER router such as Asus and Netgear instead of GL-AR750, I can access NAS without issues.
When you initially set up your NAS you most likely set a fixed IP address on the NAS itself in the subnet 192.168.1.X. This is a very common subnet for routers so switching the routers probably worked only by coincidence. GL.iNet uses 192.168.8.X subnet by default which means the 2 devices are in different subnets and can not communicate (with default firewall rules).
The easiest thing to do is to change the GL-AR750 LAN DHCP range to 192.168.1.X.