https://magnuswedberg.com/index.php?doc=OpenWRT_on_a_D-Link_DSP-W215
https://openwrt.org/toh/d-link/dsp-w215
i want to install "uhttpd-mod-lua" package.

device has internet and i can ping www.google.com so dns is ok.
when i exec opkg update -> get this errors
Collected errors:
opkg_download: Failed to download https://downloads.openwrt.org/snapshots/targets/ath79/tiny/packages/Packages.gz, wget returned 8.
opkg_download: Failed to download https://downloads.openwrt.org/snapshots/packages/mips_24kc/base/Packages.gz, wget returned 8.
opkg_download: Failed to download https://downloads.openwrt.org/snapshots/packages/mips_24kc/luci/Packages.gz, wget returned 8.
opkg_download: Failed to download https://downloads.openwrt.org/snapshots/packages/mips_24kc/packages/Packages.gz, wget returned 8.

and some more errors (new user can't post more than 8 links!)
the problem is the file "Packages.gz" not exists in the link "https://downloads.openwrt.org/snapshots/targets/ath79/tiny/packages/"!

i try to download the "uhttpd-mod-lua" package and install it using (opkg install uhttpd-mod-lua.apk) command in the directory that copied the download file.

gave this error:

Unknown package 'uhttpd-mod-lua.apk'.

Collected errors:
opkg_install_cmd: Cannot install package uhttpd-mod-lua.apk.
please help me installing this package 'uhttpd-mod-lua.apk'.

thanx
Not with what you have.
The biggest issue you're facing with that external build is that it's too old to work with anything, snapshots have eliminated opkg and are now apk only.  Your old snapshot still has opkg so is not going to work with anything, ask them to rebase and rebuild on head, then you might be able to move forward.
and copy the following text and save it as distfeeds.conf
src/gz openwrt_base https://downloads.openwrt.org/releases/23.05.5/packages/mips_24kc/base
src/gz openwrt_luci https://downloads.openwrt.org/releases/23.05.5/packages/mips_24kc/luci
src/gz openwrt_packages https://downloads.openwrt.org/releases/23.05.5/packages/mips_24kc/packages
src/gz openwrt_routing https://downloads.openwrt.org/releases/23.05.5/packages/mips_24kc/routing
src/gz openwrt_telephony https://downloads.openwrt.org/releases/23.05.5/packages/mips_24kc/telephony
if that gives error try to downgrade the version 23.05.5 to close to your snapshot
i am not sure about routing and telephony. skip those if it arises error
and for the core try
src/gz openwrt_core https://downloads.openwrt.org/releases/23.05.5/targets/ath79/tiny/packages
or in short

replace the "snapshots" in distfeeds.conf  with releases/23.05.5  or any other stable release (it would be wise to use as older ones as the date when your snapshot firmware was released)
and then try

opkg update
[update]

if you get signature  error (surely get) then edit the file /etc/opkg.conf
and remove the last line containing

option check_signature

and save it properly using nano
              
But the opkg tried to fetch from openwrt's official repository. It caused error because snapshot directory no longer contains opkg files.
which hashes does it compare to?
              
Binary compatibility cannot be assumed, and is pretty much guaranteed not to be available for anything kernel related.
tl;dr: It's not going to work, at all - unless either the original maintainer of that fork, or you yourself rebases their changes on current OpenWrt and builds a fresh image (containing everything you might want to use).
Collected errors:

opkg_download: Failed to download https://downloads.openwrt.org/snapshots/targets/ath79/tiny/packages/Packages.gz, wget returned 8.
but after that command (opkg install uhttpd-mod-lua) worked and problem solved.

i reboot the plug and it worked.
in the config file

like-
https://downloads.openwrt.org/snapshots/targets/ath79/tiny/packages
https://downloads.openwrt.org/releases/23.05.5/targets/ath79/tiny/packages