No stability issue. (Same issue with 2 devices) The connection is very unstable : it disconnects after some minutes. Then I need to disable the wifi and re-activate it to be able to browse the web again, until the next time some minutes later.

Power management for WIFI is disabled :

iwconfig | grep Power

lo        no wireless extensions.

enp4s0f1  no wireless extensions.

          Bit Rate=72.2 Mb/s   Tx-Power=20 dBm   
          Power Management:off

Workaround : use the following driver (better because muche more stable).

# Copied from https://sites.google.com/site/easylinuxtipsproject/reserve-7#TOC-The-rtlwifi_new-driver-from-lwfinger-regular-edition- (october 2018)
sudo apt-get install git build-essential linux-headers-$(uname -r)
git clone https://github.com/lwfinger/rtlwifi_new.git
cd rtlwifi_new
make
sudo make install

lsusb

Bus 001 Device 002: ID 06f8:e033 Guillemot Corp. Hercules HWNUp-150 802.11n Wireless N Pico [Realtek RTL8188CUS]

sudo lsusb -v

Bus 001 Device 004: ID 06f8:e033 Guillemot Corp. Hercules HWNUp-150 802.11n Wireless N Pico [Realtek RTL8188CUS]
Device Descriptor:
  bLength                18
  bDescriptorType         1
  bcdUSB               2.00
  bDeviceClass            0 (Defined at Interface level)
  bDeviceSubClass         0 
  bDeviceProtocol         0 
  bMaxPacketSize0        64
  idVendor           0x06f8 Guillemot Corp.
  idProduct          0xe033 Hercules HWNUp-150 802.11n Wireless N Pico [Realtek RTL8188CUS]
  bcdDevice            2.00
  iManufacturer           1 Realtek
  iProduct                2 802.11n WLAN Adapter
  iSerial                 3 00e04c000001
  bNumConfigurations      1
  Configuration Descriptor:
    bLength                 9
    bDescriptorType         2
    wTotalLength           46
    bNumInterfaces          1
    bConfigurationValue     1
    iConfiguration          0 
    bmAttributes         0x80
      (Bus Powered)
    MaxPower              500mA
    Interface Descriptor:
      bLength                 9
      bDescriptorType         4
      bInterfaceNumber        0
      bAlternateSetting       0
      bNumEndpoints           4
      bInterfaceClass       255 Vendor Specific Class
      bInterfaceSubClass    255 Vendor Specific Subclass
      bInterfaceProtocol    255 Vendor Specific Protocol
      iInterface              0 
      Endpoint Descriptor:
        bLength                 7
        bDescriptorType         5
        bEndpointAddress     0x81  EP 1 IN
        bmAttributes            2
          Transfer Type            Bulk
          Synch Type               None
          Usage Type               Data
        wMaxPacketSize     0x0200  1x 512 bytes
        bInterval               0
      Endpoint Descriptor:
        bLength                 7
        bDescriptorType         5
        bEndpointAddress     0x02  EP 2 OUT
        bmAttributes            2
          Transfer Type            Bulk
          Synch Type               None
          Usage Type               Data
        wMaxPacketSize     0x0200  1x 512 bytes
        bInterval               0
      Endpoint Descriptor:
        bLength                 7
        bDescriptorType         5
        bEndpointAddress     0x03  EP 3 OUT
        bmAttributes            2
          Transfer Type            Bulk
          Synch Type               None
          Usage Type               Data
        wMaxPacketSize     0x0200  1x 512 bytes
        bInterval               0
      Endpoint Descriptor:
        bLength                 7
        bDescriptorType         5
        bEndpointAddress     0x84  EP 4 IN
        bmAttributes            3
          Transfer Type            Interrupt
          Synch Type               None
          Usage Type               Data
        wMaxPacketSize     0x0040  1x 64 bytes
        bInterval               1
Device Qualifier (for other device speed):
  bLength                10
  bDescriptorType         6
  bcdUSB               2.00
  bDeviceClass            0 (Defined at Interface level)
  bDeviceSubClass         0 
  bDeviceProtocol         0 
  bMaxPacketSize0        64
  bNumConfigurations      1
Device Status:     0x0000
  (Bus Powered)

hwinfo --usb

03: USB 00.0: 0200 Ethernet controller
  [Created at usb.122]
  Unique ID: ADDn.6cD_jRdUh49
  Parent ID: k4bc.2DFUsyrieMD
  SysFS ID: /devices/pci0000:00/0000:00:14.0/usb1/1-1/1-1:1.0
  SysFS BusID: 1-1:1.0
  Hardware Class: network
  Model: "Guillemot Hercules HWNUp-150 802.11n Wireless N Pico [Realtek RTL8188CUS]"
  Hotplug: USB
  Vendor: usb 0x06f8 "Guillemot Corp."
  Device: usb 0xe033 "Hercules HWNUp-150 802.11n Wireless N Pico [Realtek RTL8188CUS]"
  Revision: "2.00"
  Serial ID: "00e04c000001"
  Driver: "rtl8192cu"
  Driver Modules: "rtl8192cu"
  Device File: wlx0008d395dec6
  Speed: 480 Mbps
  HW Address: 00:08:d3:95:de:c6
  Link detected: yes
  Module Alias: "usb:v06F8pE033d0200dc00dsc00dp00icFFiscFFipFFin00"
  Driver Info #0:
    Driver Status: rtl8192cu is active
    Driver Activation Cmd: "modprobe rtl8192cu"
  Driver Info #1:
    Driver Status: rtl8xxxu is active
    Driver Activation Cmd: "modprobe rtl8xxxu"
  Config Status: cfg=new, avail=yes, need=no, active=unknown
  Attached to: #2 (Hub)