HP Photosmart Wireless B109n-z will come online but not STAY online!

Question

In a nutshell: I own a HP B109n-z. It will connect on power on as it should to my main wireless access point and print from all computers on the network. After a while the printer will go offline. Web-interface cannot be reached, the printer will not respond to ping. 
To remedy the situation, turn of printer, turn on again on front switch. Printer will connect to wireless AP and print in less than a minute. The printer is currently located 30 miles away from me, so hitting on/off is not an option for me. This printer needs to just STAY online as long as access point and power is supplied. I am - as I write this - connected via remote desktop to my Win 2k8 server, and I get no response when I ping the printer - however everything else on the network works, and I actually see the printer in my routers wireless section. It is just if the printer is in some kind of hibernation.
I've read several topics about the HP Photosmart Wireless B109n-z that goes offline and it seems to be a general problem with this printer. It also seems to be a generel answer from HP forum operators to ask what router the user is using - and then stating that that particular brand of router is flawed and the newest firmware should be installed, then try again. That move seems like a stall to me.
Please - dearest HP support  - do NOT give me that answer. EVERYTHING else in my network setup works, and works ALL the time, every single day. The problem, whatever it is, clearly resides in the printer itself. All my automated daily network operations with regards to back-up and file distribution and automatic emails, plus remote operations have all been running every day for more than a year without needing maintenance. The frustration with this issue is immense.
Please help me get this solved once and for all. Help me make the printer stay online until I turn it off.
My setup:
1: Main ADSL-modem/router/wireless accesspoint is some kind of Sagem - and I can't do anything about that, it is provided by the only ISP that operates in the area (I live in Denmark). Don't know what box it is - but it uses 'Fast3504_5AD038' firmware. Broadcasts SSID, using 802.11 b + g and WPA/WPA2, running DHCP
HP B109n-z have static IP outside of DHCP range, and is placed ind same subnet as the rest of the network.
2: Attached hardware to entire network: 
One Windows Server 2008 R2 Enterprise Dell machine cabled to the Sagem box. Running 24/7/365, controlled externally  using MSTSC remote desktop every day.
One Linksys WRT54GL router cabled to the Sagem box, running DD-WRT firmware, used as a secondary wireless acces point with its own SSID. Provides wireless internet acces 24/7/364 for laptops and smartphones in the house.
Three HD digital TVs cabled to the Sagem box, receiving IP-tv signal (individually, not same channel/signal)
2 phonelines (sharing number)
The WRT54GL have a cabled connection to a Windows 7 X64 workstation, providing internet access via cabled onboard LAN from workstation and cabled connection from WRT54GL to Sagem box. Used every day, most of the day.
3 laptops connects wireless to the WRT54GL access point, which in turn - as said before - is cabled to the primary Sagem box, used every day, most of the day
HP B109n-z connects to the wireless access point given by the Sagem box without problems.
All three TVs are connected with CAT-6 cable, all other cables are CAT-5
Best regards,
A danish person

Answer 1

OK, I'll give it a shot, but ISP-supplied routers can cause lots of problems - search these forums and you will see that.
Let's set a static IP at the printer, which includes directing the printer to the DNS server, which is sometimes helpful:
Let's set a static IP address for the printer:
- Print a Network Config Page from the front of the printer. Note the printer's IP address.
- Type that IP address into a browser to reveal the printer's internal settings.
- Choose the Networking tab, then Wireless along the left side, then the IPv4 tab.
- On this screen you want to set a Manual IP. You need to set an IP address outside the range that the router automatically sets (called the DHCP range). You can find the DHCP range of the router using its internal settings page or in its manual. Use the CD that came with your router or type the router's IP address (ends in .1) into a browser.
- Use 255.255.255.0 for the subnet (unless you know it is different, if so, use that)
- Enter your router's IP (on the Network Config Page) for the gateway and first DNS. Leave the second one blank.
- Click 'Apply'.
Now, shut down the router and printer, start the router, wait, then start the printer.
After this you may need to redo 'Add a Printer' using the new IP address.
If this does not work, you can add a "standard" router (Linksys, Netgear or such) to your ISP router and turn off the ISP's wireless.   I have seen this work well many times.  You can turn the ISP's DHCP off as well, but you may have to open a port to the "standard" router.  Attach all devices to the "standard" router.
Say thanks by clicking "Kudos" "thumbs up" in the post that helped you.
I am employed by HP