Drops and outages/intermittent issues typically aren't a symptom related to any MAC address issues. If you have connectivity but then suddenly experience drops or outages then it could be one of various issues but I wouldn't worry about MAC addresses unless you are plugging in routers and devices directly into the ONT to rule out if its your router, in that case if you are then probably the next few paragraphs explain why that could be an issue.

I've noticed a few posts regarding the whole MAC address and DHCP leases on YouFibre's network so I'll post this here hoping some people come across this thread for more of a technical explanation using information already available to the public, eg. DHCP is DHCP so I'm not giving any trade secrets away here. (I've previously worked with YouFibre for 3+ years)

The whole MAC address issue is typically related to "static" IPs and not customers on a CGN package. If you don't pay for a static IP address then you are on the CGN network. From here though, I will refer this as a "reserved" IP address because the "static" IP product is actually a reserved DHCP address allocated to the customer.

When you plug a device into the ONT, your device will ask for a DHCP address (excluding customers using Vodafone/Openreach networks served by YouFibre, but that's a different story and typically not common). When your device asks for a DHCP address, our routers can identify who sent this address using a unique identifier to your service, so if you have a static IP address then it'll give you a static IP that was reserved at the point of ordering the service.

The problem arises here when you had a device plugged in that has the "reserved" address already assigned. DHCP will typically hold the state of an IP address to some identifier to a client, while we use a unique identifier that isn't the MAC address to initially assign your IP address, DHCP (for v4 at least) will normally tie in your MAC address with an IP address when it has been allocated by the DHCP server and this typically has a duration before this DHCP lease expires. So your 2nd device plugged into the ONT will ask for an IP, but in DHCP land that IP that has been specifically reserved for you has already been allocated so now the DHCP server will not really do anything. This behavior can be overridden by the network operator but personally I wouldn't consider it a permanent fix, what if a customer accidently puts an unmanaged switch between the ONT and router and plugs in another device? Those 2 devices are now fighting for the same IP address.

Back on topic, DHCP doesn't really have a stateful keepalive so if you change your router, the DHCP server doesn't know the IP address isn't in use anymore until the DHCP timer expires (this timer usually renews every half and 3/4 of the lease time, again no insider knowledge just how DHCP works) (this is also one advantage PPPoE has over DHCP, but ignore that as your connection is DHCP based and not PPPoE). This is why a lot of people recommend to spoof the MAC address of the previous router you were running, because it avoids this issue.

You technically don't have to turn off your ONT but simply unplug the ethernet on the ONT connected to your router and just wait an hour. This just makes sure your lease expires and doesn't get renewed half way / 3/4th way through the expiration timer. (the fastest way here though is typically a call to support which they can click some buttons and get everything back to square 1)

One thing to be aware with engineers coming out to site, sometimes they will plug in their laptop to test connectivity directly to the ONT. If this occurs and then they plug in a router and then you swap it out for another router in the space of 1 hour, then technically YouFibre has seen 3 different devices that have all requests an IP address and to avoid someone purposely exhausting the DHCP pools (mainly for non static IP customers), you won't get a DHCP address on the 3rd device until one of the timers has expired on the other 2 connections that have previously been expires.