I think you are correct that mt7601 at least with the mainstream driver does not support AP.
Run iw list to see radio/driver capability, in particular the supported modes.
Also if you try to configure an AP but that is not a supported mode, there would be an error logged to the system log.
ok @mk24
root@OpenWrt:~# iw list
Wiphy phy1
wiphy index: 1
max # scan SSIDs: 10
max scan IEs length: 2048 bytes
max # sched scan SSIDs: 0
max # match sets: 0
Retry short limit: 7
Retry long limit: 4
Coverage class: 0 (up to 0m)
Available Antennas: TX 0 RX 0
Supported interface modes:
* IBSS
* managed
* AP
* P2P-client
* P2P-GO
* P2P-device
Band 1:
Capabilities: 0x1020
HT20
Static SM Power Save
RX HT20 SGI
No RX STBC
Max AMSDU length: 3839 bytes
DSSS/CCK HT40
Maximum RX AMPDU length 65535 bytes (exponent: 0x003)
Minimum RX AMPDU time spacing: 16 usec (0x07)
HT TX/RX MCS rate indexes supported: 0-7
Frequencies:
* 2412 MHz [1] (20.0 dBm)
* 2417 MHz [2] (20.0 dBm)
* 2422 MHz [3] (20.0 dBm)
* 2427 MHz [4] (20.0 dBm)
* 2432 MHz [5] (20.0 dBm)
* 2437 MHz [6] (20.0 dBm)
* 2442 MHz [7] (20.0 dBm)
* 2447 MHz [8] (20.0 dBm)
* 2452 MHz [9] (20.0 dBm)
* 2457 MHz [10] (20.0 dBm)
* 2462 MHz [11] (20.0 dBm)
* 2467 MHz [12] (20.0 dBm)
* 2472 MHz [13] (20.0 dBm)
* 2484 MHz [14] (disabled)
valid interface combinations:
* #{ managed } <= 1, #{ P2P-device } <= 1, #{ P2P-client, P2P-GO } <= 1,
total <= 3, #channels <= 2
* #{ managed } <= 1, #{ AP } <= 1, #{ P2P-client } <= 1, #{ P2P-device } <= 1,
total <= 4, #channels <= 1
max # scan plans: 1
max scan plan interval: -1
max scan plan iterations: 0
Supported extended features:
* [ CQM_RSSI_LIST ]: multiple CQM_RSSI_THOLD records
* [ 4WAY_HANDSHAKE_STA_PSK ]: 4-way handshake with PSK in station mode
* [ 4WAY_HANDSHAKE_STA_1X ]: 4-way handshake with 802.1X in station mode
* [ SAE_OFFLOAD ]: SAE offload support
Wiphy phy0
wiphy index: 0
max # scan SSIDs: 4
max scan IEs length: 2257 bytes
max # sched scan SSIDs: 0
max # match sets: 0
Retry short limit: 7
Retry long limit: 4
Coverage class: 0 (up to 0m)
Device supports T-DLS.
Available Antennas: TX 0 RX 0
Supported interface modes:
* managed
* monitor
Band 1:
Capabilities: 0x17e
HT20/HT40
SM Power Save disabled
RX Greenfield
RX HT20 SGI
RX HT40 SGI
RX STBC 1-stream
Max AMSDU length: 3839 bytes
No DSSS/CCK HT40
Maximum RX AMPDU length 65535 bytes (exponent: 0x003)
Minimum RX AMPDU time spacing: 2 usec (0x04)
HT TX/RX MCS rate indexes supported: 0-7, 32
Frequencies:
* 2412 MHz [1] (20.0 dBm)
* 2417 MHz [2] (20.0 dBm)
* 2422 MHz [3] (20.0 dBm)
* 2427 MHz [4] (20.0 dBm)
* 2432 MHz [5] (20.0 dBm)
* 2437 MHz [6] (20.0 dBm)
* 2442 MHz [7] (20.0 dBm)
* 2447 MHz [8] (20.0 dBm)
* 2452 MHz [9] (20.0 dBm)
* 2457 MHz [10] (20.0 dBm)
* 2462 MHz [11] (20.0 dBm)
* 2467 MHz [12] (20.0 dBm)
* 2472 MHz [13] (20.0 dBm)
* 2484 MHz [14] (disabled)
interface combinations are not supported
HT Capability overrides:
* MCS: ff ff ff ff ff ff ff ff ff ff
* maximum A-MSDU length
* supported channel width
* short GI for 40 MHz
* max A-MPDU length exponent
* min MPDU start spacing
max # scan plans: 1
max scan plan interval: -1
max scan plan iterations: 0
Supported extended features:
* [ RRM ]: RRM
* [ FILS_STA ]: STA FILS (Fast Initial Link Setup)
* [ CQM_RSSI_LIST ]: multiple CQM_RSSI_THOLD records
* [ CONTROL_PORT_OVER_NL80211 ]: control port over nl80211
* [ SCAN_RANDOM_SN ]: use random sequence numbers in scans
* [ SCAN_MIN_PREQ_CONTENT ]: use probe request with only rate IEs in scans
* [ CONTROL_PORT_NO_PREAUTH ]: disable pre-auth over nl80211 control port support
* [ SCAN_FREQ_KHZ ]: scan on kHz frequency support
* [ CONTROL_PORT_OVER_NL80211_TX_STATUS ]: tx status for nl80211 control port support
root@OpenWrt:~#
I don't understand much 
Only phy1 have AP as interface mode.
in fact:
ph0
Supported interface modes:
* managed
* monitor
ph1 is the RPi3's built-in wifi? (I think)
Yes it looks like the dongle only runs managed (STA) or monitor, not AP.
Numbering is not certain. If you unplug the dongle and run iw list again you'll only have one radio, the built in one.
Be sure to plug the dongle back in to the exact same physical USB port, because that is part of the path stored in the config. If you plug it into a different port it will be considered a different new wifi-device.
@mk24
OK I confirm that the dongle is the PH0 so it does not support AP interface, now I wonder, before I buy another device, can I change the configuration using the dongle to receive my home wifi and use the built-in wifi as AP?
Thanks
Yes just swap the device number on the wifi-ifaces so that radio1 is the wwan connection.
Working!!!
Thank you!
How did you configured it? It is not clear to me what numbers i need to swap! 
I swapped them, and it works, but it does not give any internet connection.
you have to reverse the commands in wifi-iface, in radio1 add the 'wwan' and change option mode 'sta' instead in radio0 option mode 'ap'
Haven't seen anyone suggest running plain old 'ifconfig' to see what it thinks the devices are called for the CLI.
So, I suggest ifconfig and see if it's actually called wlan1 or something else.
ifconfig is deprecated, ip link show should be used instead.
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