Second monitor displays incorrect resolution on Ubuntu 20.04

I use a Dell XPS 13 9350 laptop. I recently purchased an HP EliteDisplay E223d monitor. Both the laptop and the monitor have native resolution of 1920 x 1080 pixels. The monitor is connected to the laptop directly via a Thunderbolt cable.*

When I run sudo lshw -C video the output is:

*-display                 
   description: VGA compatible controller
   product: Skylake GT2 [HD Graphics 520]
   vendor: Intel Corporation
   physical id: 2
   bus info: pci@0000:00:02.0
   version: 07
   width: 64 bits
   clock: 33MHz
   capabilities: pciexpress msi pm vga_controller bus_master cap_list rom
   configuration: driver=i915 latency=0
   resources: irq:139 memory:db000000-dbffffff memory:90000000-9fffffff ioport:f000(size=64) memory:c0000-dffff

A Dell page that refers to the Skylake GT2 [HD Graphics 520] video card shows that it is capable of handling up to four times the native resolution of each of my screens:

DP 1.2/eDP 1.3 max. resolution  3840 x 2160 @ 60 Hz

While a fresh installation of Ubuntu 20.04 is loading, the entire second monitor shows the purple background colour, but as soon as Ubuntu is running, the second monitor displays with a maximum resolution of 1024 x 768 px:

When used with other operating systems, the monitor has worked at full resolution.

Another issue, which may have a different cause, is that the monitor often tells me:

⚠️ Input Signal Not Found
Check the video cable and video source.
Display going to sleep.

When used with other operating systems, the monitor has worked at full resolution.

What steps should I take to get my second monitor to work correctly with Ubuntu 20.04?

From page 3 of the specifications for my laptop, I see

*Thunderbolt 3 (USB Type‑C) port Supports USB 3.1 Gen 2, DisplayPort 1.2 , Thunderbolt 3 and also allows you to connect to external display using dongles. Provides data transfer rates up to 10 Gbps for USB 3.1 Gen 2 and up to 40 Gbps for Thunderbolt 3.

Does this mean that a dongle is required, or that dongles are also supported? The monitor (sometimes) functioned at full resolution under Ubuntu 16.04, without a dongle.