I found an interesting and serious issue about wireless interface configuration on RB751U-2HnD.
After every power cycle, the router stop the boot process due to the wifi network initialization failure (I believe): the green wifi led doesn't switch on. As side effect, the router eth1 interface is completely unreachable: no wifi, no ping, no ssh, no winbox, no network connection!
I'm using RouterOS 5.14 and firmware 2.38. I configured the wifi to use 802.11 2GHZ B/G/N band (with 2GHz N the problem it's the same). If I configure the wifi card to use 2GHz B band then after 4 or 5 _quickly_ power off and power on cycles the board start working again.
In the log files I didn't find anything. I tried to use the same router configuration on a second RB751U-2HnD but the problem is the same. So I think it isn't an hardware issue.

I opened a Mikrotik support request [Ticket#2012040466000027]: I'll give you any feedback from this channel.

Does someone experienced the same issue?

Regards,
Alberto

I am having the same or similar issue. Do you have any ethernet in 2,3,4,5? I am not using my 751U for wireless at all, in fact the wireless is completely disabled. I am just using it for a simple wired only router with the wan on ether1 and a switch plugged into ether2. Every time I reboot the router, I get a solid "link" light on ether1 but it is not reachable and no "link" light on ether2 even though the switch on the other end reports a link. I have tried a more powerful power supply as recommended by MikroTik, but this didn't help anything. I have seem some other forum posts that suggest that downgrading to 5.12 will fix the problem, but that seems like a shoddy solution. Right now I have my config backed up and every time it freaks out I have to hard reset it and restore the config.

I've good news for you!
Thanks to Mikrotik's Support the problem has been detected even if not completely resolved yet.

RB751 boot problem doesn't regard wifi network card. The issue is caused by a software issue between RouterOS packeges. The issue is not currently well defined. Let me better explain the situation.

I upgraded my new RM751 to RouteOS v.5.14 installing all the packages inside the full zip download available from Mikrotik web site:
system, wireless, user-manager, ups, security, routing, routerboard, ppp, ntp, multicast, mpls, lcd, ipv6, hotspot, gps, dhcp, advanced-tools and calea

I was so excited to try the hotspot that I didn't uninstall any unused package.
All the packages installed at the same time on RB751 generate the issue.

To solve _my_ issue I simply uninstalled the following unuseful packages (I don't use them):

gps, ipv6, lcd, mpls, ups

From then on, my RB751 is working fine!

Mikrotik Support said the packages incompatibility found for RB751 will be solved in the next RouterOS release (version 5.15).

I hope these notes could be usefull.

Regards,
Alberto

v5.15 is out, please all test if the RB751U problem is fixed now

Hi,

I have the same problem too with RB751.

I have several sites using the same version 5.14 but with RB411 and RB433 without any issue.

Just wondering is this only effect RB751?

Hi,

I have the same problem too with RB751.

I have several sites using the same version 5.14 but with RB411 and RB433 without any issue.

Just wondering is this only effect RB751?

like above, upgrade to v5.15 and see if the problem repeats

I had the same problem with routerboard boot loader v.2.38 and ROS 5.xx.

Solution to fix it:
1. Downgrade ROS to 5.11
2. Upgrade (formally downgrade) routerboard boot loader to version 2.37
3. Upgrade ROS to 5.15
4. Reset configuration and load backup to the router
5. Test it!