mt7615: Unusable 2GHz radio

I have a problem with the mt7615e driver used for the 2GHz band in the Netgear r6700v2 (There's a current PR adding support for openwrt for that device).

When a device connects to the 2GHz radio, the tx/rx speeds are incredibly slow (around 0.6~1 Mbps in perfect conditions) and after a minute or so the device disconnects, and the message:
STA 00:00:00:00:00:00 IEEE 802.11: disconnected due to excessive missing ACKs
appears in the syslog.

I tried adding the option disassoc_low_ack '0' to the 2GHz wireless config and that prevents the device from disconnecting, but it doesn't solve the slow speed issue.

When I connect the same device to the 5GHz band, everything runs fine. I'm using this driver version:

> [   16.391597] mt7615e 0000:01:00.0: HW/SW Version: 0x8a108a10, Build Time: 20180518100604a
> [   16.616003] mt7615e 0000:01:00.0: N9 Firmware Version: _reserved_, Build Time: 20190103180756
> [   16.640111] mt7615e 0000:01:00.0: CR4 Firmware Version: _reserved_, Build Time: 20181207140436
> [   24.315383] mt7615e 0000:02:00.0: HW/SW Version: 0x8a108a10, Build Time: 20180518100604a
> [   24.350005] mt7615e 0000:02:00.0: N9 Firmware Version: _reserved_, Build Time: 20190103180756
> [   24.374465] mt7615e 0000:02:00.0: CR4 Firmware Version: _reserved_, Build Time: 20181207140436

Start with the mt7615e driver entry points involved in the 2GHz radio and reproduce the Netgear r6700v2 behavior while comparing it with the working 5GHz radio. Use the reported slow tx/rx rates, excessive missing ACKs, and disconnect in syslog as the failure criteria; done means normal 2GHz throughput without the disconnect.