Printer Wireless, Networking & Internet

I have recently bought the HP Smart Tank 515 printer and I am generally happy with regard to printing and scanning quality etc. The issue I have is with the wireless connection, which intermittently drops, despite all my efforts.

Specifically, I have done the following:

1. Connected the printer to my DLink router initially via the HP Smart app.
2. Set up a static IP address that is outside the router's DHCP range.
3. Set up connections to my computers and phones via the HP Smart apps for Windows 10 and iOS.
4. Tested everything works OK.

At this point the printer is connected to the wireless network - the blue light next to Wireless button is on and the LCD display shows the wireless icon at full strength (the router is fairly close). After working with the printer fine and then leaving it on but not used, I find that it disconnects from the wireless router - the blue light next to the Wireless button starts blinking and, if I try to print from any of my devices, I get an error message that the printer is offline. Pressing the Wireless button on the printer turns on the LCD display, which shows the wireless icon animating from lower to higher strength continuously. The printer does not connect to the wireless network at this point and the only way to restore the connection is to turn the printer off and on again.

I have ensured the printer firmware is updated (via HP Smart). I have experimented with the printer's network settings, e.g. turned off IPv6, turned on/off DHCP v4 and v6. I have also tried reconnecting the printer via WPS instead of network SSID and password. I have added the printer on the DHCP reservations list with its current IP address. Unfortunately, the result is always the same: the printer stays connected for a while but after a period of inactivity, the printer disconnects from the wireless router and I need to restart it to connect again.

I would appreciate any help or comments.