When I used the wl-500gP with mediatomb in the back-days, I wasn't satisfied at all. The indexing of video worked, but i took ages. You may run mediatomb with -D and -l {logfile} options while doing a rescan so we're may be able to understand whats wrong there.
I suggest using minidlna there is no hazzle when importing video. You loose transcoding-ability but in my eyes this is useless on a soho-router anyway.
RT-N66U TomatoUSB 1.28.0000 MIPSR2-135 K26AC USB AIO-64K running Entware-NG
pyload 0.4.9
minidlna 1.1.2
samba 3.6.5
openvpn 2.3.10
̶W̶L̶5̶0̶0̶g̶P̶ ̶1̶.̶9̶.̶2̶.̶7̶-̶r̶t̶n̶-̶r̶4̶7̶5̶0 retired