how to enable Android apps to browse user mount points in /mnt
Through my research, I couldn't find a way to create a symbolic link in '/sdcard' (my android internal storage) from /mnt/routersd (mountpoint to //192.168.x.x/sdcard); as filesystem for /sdcard in android doesn't support symbolic links.

I also tried busybox mount -o bind /mnt/routersd/ /sdcard/mylink but mylink couldn't be browsed by the said apps (those apps find mylink as an empty directory).

Eventually, I created the symbolic link in my external sdcard, namely sdcard1 (NTFS v3.1), successfully (recent NTFS versions support symbolic links); as follows :
busybox ln -s "/mnt/routersd/" "/storage/sdcard1/mylink"

As Adobe Reader, WPS Office, and many other apps can browse into external sdcard, mylink can be visible in their dicrectory list happily. Now, I can successfully browse my remote files and work on them from within the said apps.