Installing Ubuntu 16.04 on a Dell Inspiron 14

tl;dr version: To get Ubuntu installed on the Inspiron 14 with the Realtek wifi working, leave the machine's setup as it is by default (UEFI boot mode, Secure Boot enabled), run the Ubuntu installer, connect to wifi when that's offered, but don't accept the option to install third-party drivers for graphics and wireless.

The longer version....

This evening it took me three attempts to get Ubuntu 16.04 installed onto my friend Dave's Dell Inspiron 14-3452.

With a Celeron chip and 2GB RAM, this cheap and cheerful machine was sluggish under the load of the Windows 10 that it came with, so he asked me to wipe that and install Ubuntu, which is already on his desktop machine.

The only networking offered is a Realtek wireless chip. There's no Ethernet.

The machine has no optical drive, and I happened to have an Ubuntu 16.04.1 LTS disk already burned onto DVD, so I booted that from an external DVD drive.

First time round I took the "Try Ubuntu" option to boot into Ubuntu and check that everything would work until Linux. The Unity desktop started up, and the graphics worked fine. I clicked on the network icon and was pleased to see that lots of wireless access points were visible, so I assumed that the wifi was fully operational. Wrong there!

Falsely reassured, I ran the installer, declining the options of downloading updates and installing third-party drivers for graphics and wifi. My reason for declining the updates during installation is that I generally find that it's quicker to install offline, and then run 'apt update' on the installed system. I also reasoned that since the graphics and wifi were apparently working, there was no need to install third-party drivers.

When I booted into the installed system, I selected my wireless access point, entered the key, and was disappointed to find that it failed to connect. I ran lsmod and noted that the following wireless-related modules were loaded: rtlwifi and rtl8723be. I did some research and found this page...
https://askubuntu.com/questions/6356...s-card-to-work
...on which Pilot6 had kindly supplied links to an alternative rtiwifi driver and dkms .deb packages. I copied these across to the Inspiron 14 via flash drive. While installing them, Ubuntu presented me with a warning about installing third-party packages on a Secure Boot enabled system. I took the only option which was available if I wanted to continue installing the drivers, which was to turn off secure boot and create a security key which should then be entered when the system restarts. I rebooted, and was confronted with a screen (from the firmware, not Ubuntu) which presented options to 'enroll' a key or hash. There was nowhere to enter the password which I had created. I explored the 'enrolling' options but found nothing helpful.

So next I thought I'd reinstall with secure boot disabled in setup. I did so, but on reboot was presented with a GRUB prompt (not menu). I tried to find a kernel to boot via the GRUB command line but failed.

Next I went back into setup, switched boot mode to Legacy, and tried another reinstall. That also failed. (Can't remember why -- should have kept better notes!)

Finally I re-enabled UEFI mode and Secure Boot, went for a reinstall and chose the option of connecting to wifi during the install. It successfully connected to my wireless access point without any whingeing about needing to disable Secure Boot or setting a password. I continued the install, declining the offer of updates and third-party drivers. My hope was that the successful wireless setup would 'stick' when I rebooted into the installed system. And I'm glad to say it did!