Configuring OpenWrt from cli isn't difficult, but if you can avoid that -for now- and just get it working quickly, that's probably preferable.
The good news, you have apparently successfully flashed OpenWrt on the device, without bricking it - that's always a good start.
You have basically two choices here:
- using the (online-) imagebuilder, to generate an image including luci-ssl
- temporarily putting the dl-wrx36 behind your old router, but in front of your computer
[modem] <--> [old/ working router] <--> [dl-wrx36] <--> [computer]
just make sure to avoid an overlap of your subnets, so neither modem nor old/ working router should use the 192.168.1.x/24 subnet.
- OpenWrt defaults to DHCP-client on WAN, so this should give you internet access, both on the router itself and to your computer behind it, this means opkg should work -->
opkg update && opkg install luci-ssl should succeed and give you luci access
If you are familiar with the imagebuilder and have successfully used it (for other devices) so far, this is the easiest option, but if you aren't, I'd play it safe (first) and use the second approach (get familiar with the device and its behaviour first, before 'risking' to flash an untested custom image, which might miss important packages).
If the second approach would require too much trouble (changing the IP/ subnet of your old router causing too much conflict), you can try a third option.
Cable ISPs usually lock onto the WAN MAC of the connected router and won't accept a new device easily, either you have to clone the WAN MAC address of your old router on your dl-wrx36's WAN (not difficult, but this implies editing /etc/config/network with vi or indirectly through uci, neither of which is particularly beginner friendly to use) or you teach your ISP about the new MAC address of your dl-wrx. The easiest way to do this, is:
- power off the cable modem completely and to let it sit powered off for 5-10 minutes
- connect the dl-wrx36 in the mean time (powered off as well)
- power up the cable modem and let it fully boot up (at least 5-10 minutes)
- power up the dl-wrx36 and let it fully boot up (up to 5 minutes)
- now check if you get a connection, in many cases this works - in others you might have to leave the cable modem powered off for longer periods of time (~an hour or ~over night), in some cases you might have to call your ISP to manually release the old MAC address (in which case the lazy/ impatient me would go one of the previous routes and clone the MAC address of the old router)
Once you have internet access (so ping working) to the dl-wrx36 one way or another, just opkg update && opkg install luci-ssl and you're set, at this point there is no material difference to using a stable release anymore (aside from snapshots and ~packages being a rolling release, meaning you might have to sysupgrade to newer snapshots, if you want to install additional packages later on).
That aside, the dl-wrx36 is supported officially in stable since 23.05.0, so you could have gone with the stable release instead of using a snapshot. However, given that the (sub-)target name has changed between 23.05.x and snapshots, I wouldn't really suggest to downgrade your snapshot to stable now - it's not difficult, but involves a minimal-but-non-zero risk, so get it working first - reconsider later (if you are on a qualcommax/ipq807x snapshot, I'd stay there - if you are on ipq807x/generic, you can easily downgrade to 23.05.2; ubus call system board should tell you and/ or the name/ URL of the images you downloaded/ flashed).
Disclaimer: Depending on your familiarity with OpenWrt, there are multiple alternative approaches, but I'm always a fan to go slow, to be careful with new 'unknown' devices, to take baby steps and to start with the least risky options first, before taking the big hammer,