Check the cabling, patch port, switch port to the affected AP. Have seen it where the AP is powered up and in Mesh mode but not able to operate as a gateway, turned out to be cabling that had degraded. Try rebooting the ap before you do that
If your access point is coming online as a repeater then it’s most likely it can’t get an IP address via DHCP on the Ethernet connection. So check to make sure the cables are good, and that your subnet/VLAN and DHCP are correctly configured (and that there are spare DHCP addresses available). I don’t believe configuring a static IP address will work if the access point is running as a repeater.
I'm curious as to what the WAP is plugged in to. Most connectivity issues (working fine, then not with no change) with WAPs come down to a bad wire, a bad switch, or a bad WAP. I had one WAP that would randomly go to repeater mode and then back. Cable was fine, I re-tipped the ends, tried a different port, tried a different Meraki WAP, and always got the same result. I put a different brand WAP in place of it, and within a day I was alerted that the WAP had dropped down to 100mb from the 1000mb. I replaced the Cisco POE switch (SG300-28MP) a different brand of POE switch and haven't had a problem with that WAP since. Meraki does a bad job of defining why a WAP isn't functioning properly in many of these cases.
I've found that the model of switches that I inherited are junk, and have had 3 fail in the last month. Cisco reliability and stability ain't what it used to be!